Bright colour, smooth scales, eyes that aren't protruding, fins that look battered could mean fin rot, a bit of curling isn't too bad though,could be born with it.
Hel be quite active poking around all the nooks and crannies, they will spend a lot of the time near the surface to breath but they also go down low and anywhere there's a gap, they tend to start and stop as they rummage about, but slowly, if he's darting around it could be a sign of stress, and If hes lying on the bottom there could be more serious problems obviously.
If he's in with other fish you could watch him for temperament and potential tank mates, eg I got mine from a tank with shrimp, so I got the shrimp too as I could see they were ok with each other. But shop around youl find one you like and youl know when you see him.
Hope this helps =)