Hi TP,
It would make sense if you could test the mattress/topper in person and if the store carries a topper that would be a good “match” for you in terms of PPP in combination with the mattress but if you can’t then there is always some additional uncertainty and risk in purchasing a topper where you can’t test the combination in person before a purchase in which case the exchange/return policy of the store you buy from can be one of the most important parts of the “value” of a topper purchase just in case the topper doesn’t turn out as well as you hoped for (see post #2 here).
On the other hand (and as they mentioned) … if a mattress/topper combination is a good “match” for you in terms of "comfort and PPP then it also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is very likely because the upper layers or a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
I don’t have any specific suggestions because the height of your sleeping surface is just a preference choice and outside of your own preferences and your ability to get in and out of bed easily one height isn’t any better or worse than another.
If you add up the height of all the support components you would be using under your mattress (bedframe and foundation) and then add the thickness of the mattress you would have the height of your sleeping surface and you can compare it to your current mattress and support system (or others that you are familiar with) as a reference point and decide if you would prefer something that is the same or lower or higher.
Once you have made your purchase then it would be somewhat difficult to either raise or lower the height of your sleeping surface because either one would generally involve exchanging the bedframe you are using for another one that is a different height (either a regular or low profile steel foundation for example) or replacing the foundation for one that is a different height (either a regular or low profile foundation or a bunkie board).
Having said that … it would probably be a little easier to add some additional height because outside of using a taller foundation, you could add some risers under your bedframe (see some examples here), or you could add some taller legs directly under your foundation (see some examples in post #8 here), or you could also add an ultra firm foam layer on top of the support system and under your mattress.
Phoenix