Hi malex,
Welcome to the Mattress Forum! 
You’re welcome!
Latex will certainly be a good quality of material to choose and will offer an extended comfort life, and also does tend to provide some of the “push back” that you mentioned you’re looking to find.
The thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials (type, firmness etc.) and on all the other layers in the mattress. Thickness is only one of many specs that are used to make different mattresses that perform and feel differently and that makes a mattress suitable for one person and not another. There is more about the effect of thickness in post #14 here. Regardless of how thick or thin a mattress may be … the most important part of the “value” of a mattress is how suitable it is “as a whole” for your particular body type, sleeping positions, and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) regardless of how thick it may be. There is certainly no reason that at your weight you would “need” more thickness than either of the mattresses you are considering but you may still “prefer” a mattress that is thicker or thinner depending on the design.
When you’re not able to test a product in person, the best thing that you can do is have an extended phone call with the retailers/manufacturer, as they can best speak to the products that they offer that they feel will be the best fit for your sleeping style, BMI, preference and price range. An all-latex 7" mattress certainly can contain enough support for someone of your BMI. It was common in years past for people to sleep upon 5" or 6" slabs of solid Dunlop latex. Those tended to be quite firm. In the mattress you just mentioned, a firm Dunlop base layer with a softer Talalay layer on top could be a potential combination for a side sleeper of your size. I believe that is what SleepEZ would recommend.
This mattress uses a polyfoam core wit 2" of latex on top. Choosing an item using a polyfoam core would be one way to keep your budget in a lower range, but you would be sacrificing some of the qualities of having an all-latex product. You can see some of my comments about Plushbeds in post #2 here. A forum search on Plushbeds (you can just click the link) will bring up much more information and comments/feedback about them as well. They also use good quality and durable materials in their mattresses (natural or organic Dunlop and Talalay latex) and there are no lower quality materials or weak links in their latex mattresses that would be a cause for concern either and they would certainly be a much “better than average” choice compared to most of the mainstream mattresses that most people end up buying. They do a lot of internet advertising so they do attract a lot of online attention for those that are looking for a latex mattress but I would make some careful “value comparisons” with some of the other component latex options that are available to you because they may not be in the “best value” range for many of the members here that are aware of the many other similar options that are available to them.
The 7" would be more along the lines of the previous SleepEZ model you mentioned, but the Flexus is all Dunlop (the SleepEz you mentioned has Talalay on top which some people find a bit more “contouring” than plush Dunlop). The 9" would offer more choices in configuration and also would have a choice of Talalay on top.
This is different from the other SleepEZ mattress you mentioned earlier and is flippable. It is all Dunlop and would not have the option to be made as “plush” as the 7" Natural Latex Mattress 7000.
[quote]Latex Mattress Factory (LMF)
latexmattressfactory.com/products/natura…?variant=17631290689[/quote]
This is a one-sided mattress using Dunlop latex. It has a 6" Dunlop core than can be chosen in plush, medium or firm, with 1" of wool on top in the quilt. It is similar to the SleepEZ Naturale, except that it isn’t flippable like the SleepEZ. This LMF mattress is often used as a base in combination with a softer latex topper.
[quote]Pure Green by Sleep On Latex 9’
sleeponlatex.com/products/natural-latex-mattress[/quote]
This model is similar to the LMF, except that it is 9" instead of 7", and it adds a 2" soft layer on top of the 6" core (all Dunlop). You can choose from a soft, medium or firm base Dunlop piece.
You’re certainly looking at items that use good quality components, and they are using all layers of latex (with the exception of the Plushbeds).
SleepEZ, Flexus, Latex Mattress Factory, and Sleep On Latex are all members of this site which means that I think very highly of both 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’re considering products like this online, 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 (latex, resilience, firmness etc…) and the options they have available that may be the best “match” for you based on the information you provide them (5’ 4", 145 lbs., side sleeper), 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, options, and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else. I would trust their recommendations. An online retailer will generally suggest a mattress that they believe will have the best chance for success (based on averages) since a return is very costly to an online retailer or manufacturer and it’s in their best interests to make a suggestion that they believe will have the least possible chance of being returned.
If you aren’t confident that an online mattress will be a good match for you, then the return/refund policy and any costs involved would become a much more important part of the “value” of a mattress purchase (as you already alluded) so you can try out the mattress in your bedroom instead of a showroom with the only risk being the cost of returning a mattress if in the worst case, and in spite of the “best efforts” of everyone involved (both you and the retailer you are buying from) the mattress doesn’t turn out to be as suitable a choice as you hoped for.
The good news is that you’re considering some products using components of higher quality. I think your next step would be to make a PROS and CONS list for each item based upon your personal value equation and make a few more phone calls with the manufacturers.
I’ll be interested to learn about what you decide to do.
Phoenix