Hi Ciontentt,
I see you’ve already found the post that I would have linked to answer your question 
Unless you can test the mattress/topper combination I would tend to buy a mattress that you can test in person that already uses good quality materials in the comfort layers (pillowtop or otherwise) rather than buying a firm mattress that may have some “questionable materials” in the mattress and then going through the uncertainty of choosing a topper that is a good match for both the mattress you are using it on and for you.
The best place to start is the mattress shopping tutorial here which has all the basic information, steps, and guidelines you will need to make the best possible choice … regardless of your budget range.
Phoenix