Hi Vicky,
This would certainly be a good quality/value choice compared to other similar mattresses in its budget range that would be available in the mainstream industry and it’s also two sided which means that it would be more durable than a similar one sided mattress as well as long as you rotate and flip it on a regular basis (see post #3 here).
In theory you can use a topper on any mattress that is still in good condition and doesn’t have any soft spots or visible impressions and just needs some additional softness and pressure relief but your own testing or personal experience is the only way to know whether a topper would be “too much”, “just right”, or “not enough” and whether a topper would be a good match for you on a specific mattress in terms of PPP.
If the only lower quality material in a mattress is 1" of lower density polyfoam in the quilting then by itself it wouldn’t be a “weak link” in the mattress (see the guidelines here) but if there are other lower quality materials in the comfort layers as well so the total is more than “about an inch or so” then it certainly would be a weak link in the mattress in terms of durability. If you can find out the information listed here and post it on the forum I’d certainly be happy to make some comments about the quality and durability of the materials and the mattress as a whole.
There is no specific number that would apply in general because each person and mattress would be different and it would depend entirely on the specifics of the mattress and the topper and on the body type and sleeping positions of the person. The only way to know whether any mattress/topper combination is a good match for you in terms of PPP is based on your own testing and personal experience.
I would tend to avoid buying a mattress with the intention of buying an additional topper where you can’t test the combination in person unless you have no other options available because of the additional uncertainty and risk involved (see post #2 here) compared to buying a mattress that doesn’t need a topper that you can test in person.
If you do buy a mattress that unintentionally turns out to be too firm then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline for choosing the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success and also includes a link to a list of some of the better sources for toppers I’m aware of as well but I would use this as a “backup” just in case a mattress you purchase doesn’t work out as well as you hoped for and needs some additional softness.
If this is happening on every mattress at every possible firmness level available (some mattresses would be softer than some mattress/topper combinations) then it would generally be pointing to some kind of physical or health issue rather than an issue with the mattresses themselves.
Phoenix