Hi fluid_bwt,
You can see my thoughts about buying a mattress/topper combination where you can’t test the combination in person in post #2 here.
If you buy a “cheap” mattress to use with a high quality topper then it’s very likely that there will be several inches of low quality materials in the upper layers of the mattress that could be a significant weak link in the mattress because most cheap mattresses use some very low quality foam that is not very durable in their comfort layers. A mattress that uses high quality materials in the comfort layers (which would become transition layers if you add a topper to the mattress) and doesn’t have any significant weak links in the design isn’t likely to be “cheap”.
I would also make sure you’ve read the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choices and it also includes some links to some of the better online options I’m aware of that may also be worth including in your research.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses.
Phoenix