I agree with *not* getting a firm mattress. I got one years ago, thinking the firmer the better, and didn't sleep well on it because it was so firm my body would develop pressure points of soreness. Also agree with *not* getting a pillowtop. The pillowtop will develop compressed spots from your body long before the mattress gives out.
It was years ago when we shopped, but I think it was the King Koil that my husband didn't like because he could hear the springs creak when he moved on it. It didn't bother me, but it bothered him a lot. I know it sounds odd, but pay attention to the sound when you move around on the mattress.
Usually the cost savings from using a prior box spring isn't worth it, and I believe most mattress companies won't warranty a mattress that is not used with a new box spring. If you read the warranties closely, though, I'm not sure how much they're worth. The mattress usually has to show at least a 1" slump plus, as you know from the recliner experience, it's not easy to return a big item like a mattress. Better to get it right the first time.
Edit: I found this website about King Koil that might be useful:
http://www.sleeplikethedead.com/matt...king-koil.html
Also, I do recommend getting a flipable mattress (two-sided). Our current one is not; our previous mattress was, and in my opinion was a lot better and lasted much longer.