Hi suspent,
The first place I would start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that you will need to make the best possible choices … and know how and why to avoid the worst ones including any of the major brands such as Sealy (see the guidelines here)
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 PPP (Posture and alignment, Pressure relief, and 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 (or that doesn’t feel as close to another mattress as you hoped it would) that are involved in each of them 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, durability, and value.
There is more about the many variables that combined together 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 her in a comfortable temperature range.
There is also more about gel foams in post #2 here and while they can have a temporary effect on sleeping temperature when you are going to sleep at night, once temperature equalize then the insulation properties of the foam (particularly memory foam) become dominant and the cooling effect doesn’t generally last over the course of the night.
I would also be very cautious about using coil counts as a reliable or meaningful way to choose an innerspring mattress (see this article) and if she prefers an innerspring mattress I would make sure that the materials above the innerspring (which will be the weakest link in the mattress in terms of durability) meet the quality/durability guidelines here.
Once you reach step 3 in the tutorial if you let me know your city or zip code I’d be happy to let you know about any of the better local options or possibilities I’m aware of in your area.
The tutorial also includes several links to some of the better online options I’m aware of as well (in the optional online step)
Phoenix