Hi swarm,
If you decide to buy a firm mattress and then add a topper 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.
While the most important part of the “value” of a mattress purchase is comfort and PPP (Posture and alignment, Pressure relief, and Personal preferences), outside of PPP a mattress is only as good as its construction and the quality and durability of the materials inside it and the durability and useful lifetime of a mattress is also a very important part of the value of a mattress purchase. No matter how it may feel in a showroom … there would be little value in buying a mattress where the use of lower quality and less durable materials leads to foam softening and breakdown and the loss of comfort and support much too quickly after a purchase and the loss of comfort and support it isn’t considered to be a defect and isn’t covered by mattress warranties (see post #174 here). This is certainly a choice that I would avoid.
Phoenix