Pay attention to the depth of the mattress.

There are some that combine the function of both mattress and box spring. These can be 18 or more inches deep, and take specially made sheets that won't be available in local stores.

These also cannot be flipped, only rotated.

Edit: I have one of these, while its been comfortable and has lasted longer than the last two, I'd likely look for a std mat and box set. Mostly because of having to special order the sheets.

Edit2: that depth can also make your blankets and spreads not quite reach the floor any more.