I couldn't find a cotbed with drop sides (sure they exist, but couldn't find one)
Underbed storage is handy
I wouldn't bother with a cot top changer unless it's for a very small room and you're limited on space (just get a chest of drawers and put a change mat on that, or change on the floor)
3 different heights is essential though, IMO. More so that drop sides (which I only used when I was changing the bed as the noise of lifting it back up woke DS.
So.. silent (or near enough) drop sides if you can find them, otherwise don't bother.