King Koil Contourpaedic latex mattress has dipped

At the beginning of 2001 we bought a King Koil Contourpaedic Latex mattress, which now has two distinct hollows in it, and which needs replacing, but are completely befuddled as to our needs. We always fancied a latex mattress but they're a bit expensive, so now we are looking at pocketed springs. We are not young, I am 8 stone, have back problems and fibromyalgia, which means painful muscles and ligaments. My husband is 11 1/2 stone with hip problems. Could you please advise as to the degree of firmness and number of pocketed springs which would be best for us? We don't want the mattress too firm, but do need a firm edge to it. Any recommendations would be gratefully received.