Hi jAri,
I think that your idea makes a lot of sense and would be what I would be leaning towards if I was in your situation as well.
The best place to start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choice … and more importantly know how and why to avoid the worst ones.
Since you will be using the mattress most of the time I would also lean towards retailers/manufacturers that carry both mattresses and toppers so you can test the mattress/topper combination in person to make sure that your “sleeping system” is a good “match” for you in terms of PPP (see post #2 here).
If you do decide to go in this direction and if you can’t test a specific mattress/topper combination in person (and are buying the topper separately after purchasing the mattress) then post #2 here and the topper guidelines it links to has more information that can help you use your experience on a mattress as a guideline and reference point for choosing the type, thickness, and firmness for a topper that has the least risk and the best chance of success.
If you let me know your city or zip code I’d be happy to let you know about any of the better options or possibilities I’m aware of in your area.
Phoenix