Hi mallinds,
Welcome to the Mattress Forum! 
I’ll do my best to break down your post and answer some of your concerns.
Congratulations! You must be excited.
When looking at mattresses, there are no “standard” definitions or consensus of opinions for firmness ratings (and pillowtop isn’t a comfort designation – just a manner of sewing a mattress together) and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science. In the end, your own personal opinion will be the best guide as to the level of plushness that you desire.
The materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material. There is much more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range. There are many variables involved in your sleep environment temperature, including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on sleeping temperature as the type of foam in a mattress) and where you are in the “oven to iceberg” range. Latex will tend to be more breathable than polyfoam, which will tend to be more breathable than memory foam, although all foams are insulators to an extent. A product with an innerspring can also move more air than one using multiple layers of foam. Also, sleeping “in” a mattress more (like a very plush mattress) can tend to sleep warmer than a mattress you sleep more “on” (a harder feeling mattress), as the plush mattress will allow for less heat exchange and also be more insulating.
Post #3 here has more information and suggestions about heavier weights that is worth reading. A high BMI presents special challenges and generally requires firmer materials (in the support layers especially). This could be firmer latex or innersprings (the type of support component would be a personal preference and in the right design either could be suitable) or even a zoned construction. The same overall guidelines apply with higher weights though that PPP (Posture and alignment, Pressure relief, and Personal preferences) along with using high quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing. Post #14 here also has more about the benefits of thicker comfort layers and thicker mattresses for higher BMIs.
You’ll definitely want to make sure that your deep support within the mattress is adequate for your BMI, as alignment is the first key thing I recommend to defer toward. You can choose to go with something a bit softer on top, but per the guidelines I linked to previously, you’ll want to make sure those materials are durable and substantial enough to provide you a good comfort life.
The process for selecting a mattress is the same for those of a low or high BMI, so I’d suggest you start your research by reading the mattress shopping tutorial here and two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
In its simplest form … choosing the “best possible” mattress for any particular person really comes down to FIRST finding a few knowledgeable and transparent retailers and/or manufacturers (either locally or online) that sell the types of mattresses that you are most interested in that are in a budget range you are comfortable with and that you have confirmed will provide you with the all the information you need about the materials and components inside the mattresses they sell so you will be able to make informed choices and meaningful comparisons between mattresses and then …
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Careful testing (hopefully using the testing guidelines in the tutorial) to make sure that a mattress is a good match for you in terms of “comfort”, firmness, and PPP … and/or that you are comfortable with the options you have available to return, exchange, or “fine tune” the mattress and any costs involved if you can’t test a mattress in person or aren’t confident that your mattress is a suitable choice.
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Checking to make sure that there are no lower quality materials or weak links in a mattress you are considering relative to your weight/BMI range that could compromise the durability and useful life of the mattress.
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Comparing your finalists for “value” based on #1 and #2 and all the other parts of your personal value equation that are most important to you.
If you’re considering online mattresses, you may wish to use the experience and expertise of the site members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone for high BMI applications that can help you make good choices. There are a wide range of latex and hybrid options included in the choices there that could work well for your situation, and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
If you do decide to investigate items online, then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are of a “similar” BMI. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
I know this is a bit of reading, but I think it will help provide you a framework to make an educated mattress choice. Let me know if you have more specific questions after you’ve had a chance to review this material, and I’ll do my best to be assistive.
Phoenix