Hi jsj5g,
Welcome to the Mattress Forum! 
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
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 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 he will sleep), durability (how long he 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).
While again nobody can speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress … 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 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. Shopping by brand is not a way to shop, but instead look at the componentry within the mattress itself.
The Novosbed medium uses multiple layers of 5 lb. memory foam and a 1.8 lb. polyfoam core, and those are good materials for your BMI, but over 200 lbs. or so I would recommend a core of at least 2.0 lbs.
The Dreamfoam Ultimate Dreams Eurotop uses 3" of Talalay latex and a 2.0 lb. polyfoam core. This is good quality material for your BMI.
The Dreamfoam Ultimate Dreams Supreme Gel uses multiple layers of 4 lb. memory foam and a 1.8 lb. polyfoam core. The 4 lb. polyfoam is the minimum I would recommend and I would use caution for those approaching 200 lbs. or so.
The Dreamfoam Ultimate Dreams 13" uses layers of 4 lb. and 5 lb. memory foam, and a 1.8 lb. polyfoam core. The 4 lb. memory foam is again something I’d advise caution about for those approaching 200 lbs. or so.
The Rocky Mountain latex mattress comes in different configurations and certainly would use good and durable materials. You don’t mention which memory foam mattress in which you had an interest, but you can certain use the durability guidelines I linked to earlier to rate the quality of the materials used I whatever mattress you choose.
Dreamfoam, Novosbed and Rocky Mountain Mattress are all members of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. I certainly wouldn’t hesitate to purchase a mattress from any of them.
When you can’t test a mattress in person 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 similar to you. 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.
A good online retailer or manufacturer will generally make suggestions that they honestly believe have the best chance of success based on the information you provide them when you talk to them on the phone because this is in both your own and their best interests but again … at the end of the day the only way to know for certain whether any specific mattress is a good match for you in terms of comfort, firmness, and PPP will be based on your own careful testing and/or your own personal experience so if you can’t test a specific mattress in person then the options you have available after a purchase to either exchange the mattress or individual layers or components or return the mattress for a refund (and any costs involved) would generally become a more important part of your personal value equation just in case a mattress you purchase doesn’t turn out as well as you hoped for.
Many manufacturers, especially online, will offer methods for manipulating the comfort of a product, often within a certain timeframe, should you find that your initial selection unfortunately doesn’t work out as you had hoped. Adding a comfort layer or exchanging out a layer doesn’t impact durability of a mattress, as long as the material that is exchanged is of equal or higher quality to that it replaces.
As you are looking online and I would use the experience and expertise of the 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 that can help you make good choices. There are a wide range of latex and memory foam and other options included in the choices there and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency. There are many choices available there in your price range.
Phoenix