From what I've read on another site those SeaNymphs have a tendency to develop stress cracks in the bottom ribs so look it over closely. Also check along the seams at the rivets as there have also been many with cracks there.
As to boat size, 16' is about as small as one would want if regularly fishing with 4 adults and to be honest if it were me I'd be looking for one of the larger 16' Starcraft, Sylvan, Smokercraft, Alumacraft or Lund's.
I use a 16' Alumacraft Classic and 3 adults gets cozy. Then it also depends on what type of fishing you are going to be doing.