Hi Goldfish_7,
Unfortunately you are not the only one that found the forum that was facing the same difficult choice (See post #66 here and two other posts that are linked there as well). Part of the difficulty as well is that one of your options may be choosing a less expensive mattress that has a good support system (on the firmer side because of your weights) with as little in the comfort layers as possible (reducing the possibility of lower quality foam softening and degrading) so that you can add your own comfort layer in the form of a topper that uses more durable material, offers choices in terms of firmness level and thickness, and can be replaced separately if it wears out or softens faster than the mattress support or transition layers. Because this type of mattress is generally less expensive and the store may not offer toppers that you want … you may be trading a more expensive mattress for a less expensive one and any difference may not be refundable so this may also need to be taken into account in your choice. In other words … what would they offer you if the mattresses you wanted to use underneath a topper is only half the price of what you paid.
The choice is made even more difficult because of the lack of information that is available about the materials in all the brands they sell and the ingredients of the mattress (or the lack of them in the case of the polyfoam comfort layers) are much more important than the brand when you are trying to choose the best possible base to use underneath a topper.
If you could let me know the store you are dealing with and they have information about what they offer available online I’d be happy to take a look and see which option or mattress may be the “least bad” choice available to you. This would be based on what was in the mattress rather than the brand itself (there really isn’t a major brand that is “less bad” so much as specific mattresses they make that may be more suitable for what you need).
Phoenix