ANYTHING as long as it's not a toddler bed!!! (Seriously, how much of a marketing ploy are THOSE?) A simple waterproof matress cover (we have 2, for "flu" cases) takes care fo the whole potty training thing.
We chose to go with a full size bed so we can lay down with him (or make blanket tents, read in bed, etc.) and so that he'll be comfy in it over the next 10 years.
We went with a GOOD bed (beautyrest), under the theory that growing bones and muscles need good support (even though kiddo is a KID and therefore doesn't really care/notice)... but got it super cheap ($200 for bed and box, instead of $800) by going through an outlet & buying last year's model. We didn't bother to buy a frame for it for 2 years... just kept it on the floor. Once we got a frame we got one with a trundle... which has worked out like a dream for sleepovers now that he's 8.
((BTW... to be completely honest... we had an IKEA bunkbed as a temporary solution while in the middle of moving... bed was 4' off the ground and the bottom was a "fort". It was for while we were in between houses and had NO spare room whatsoever. Made night time potty training, um, interesting. We'd intended him to sleep on the bottom, but he wouldn NOT. He just loooooooved the top, and would melt down if required to sleep on the bottom, and not sleep (although he slept fine up top). So I'd avoid a bunk.))