Hi tidoubleger,
Just in case you haven’t read it yet … I would make sure you’ve read the tutorial post here which has all the basic information steps and guidelines that can help you make the best choices.
A topper can be a good way to soften a mattress that is too firm and is still in good condition with no soft spots but it’s not usually an effective solution for a mattress that is too soft and could make the problem worse because a firmer topper will just sink into the softer materials below it. There is more information and some ideas about mattresses that are too soft or are sagging in post #4 here.
I would tend to avoid any memory foam mattress that doesn’t meet the criteria in post #10 here because of durability issues. I would also tend to avoid buying a mattress / topper combination where you aren’t able to test the combination in person and focus on a mattress that is as close to ideal as possible by itself. Buying just a mattress that is a good match for you in terms of PPP can be difficult enough by itself and having two unknown variables to purchase (the mattress and the topper) can be more complicated than just buying a mattress by itself. It can also be more costly than buying a mattress where the same material that is in the topper is already in the mattress. If you do decide to go in this direction then I would buy the mattress first and use your actual experience on the mattress and the topper guidelines in post #8 here as a reference point for which topper may work best for you.
There are also many versions of memory foam and gel memory foams and some are warmer/cooler than others. There is more about cooling down memory foam in post #6 here. There are also many factors involved in the sleeping temperature of a mattress and it’s often a combination of factors that leads to sleeping warm. There is more about temperature regulation in post #2 here.
If you let me know which city or zip you live in I’d also be happy to let you know of any of the better options or possibilities I’m aware of that are close to you that you can test different types of mattresses in person which can be an important part of a successful purchase regardless of whether you end up buying locally or online.
Phoenix