Hi Carolinagrl,
Welcome to the Mattress Forum! 
You are correct that the specifications you provided provide no meaningful information about the foams within the mattress you’re having replaced.
I’m not completely sure about what you’re asking. If you’re getting a replacement mattress, then I would suggest sleeping upon that to see how it feels before potentially modifying the product with a topper. The issue is that if your base mattress is unsupportive or uses lower quality foams that develop deeper body impressions, a topper won’t address that situation, as it will only be able to conform to the surface upon which it is placed. You’ll also want to make sure that the foundation upon which you place your mattress is firm and flat.
Additionally, as you sleep prone some of the time and are a 30 BMI, you’d generally desire a bit of a “firmer” sleeping surface (as you indicated), so if you added any extra padding material, you’d generally want to focus upon higher ILDs and in layers that are not too thick. A firmer latex or firmer high-density polyfoam could be good options. There is more information about choosing a topper in post #2 here and the topper guidelines it links to.
Phoenix