Hey.. you might also want to just ask him if he minds you looknig at them.... bring a wrench and take out the plug.. see if you got spark and compresion... you can worry about gas later...
But you might find that one or two are total screwed.. might be why he has three of them... probly got them to make one out of and didn't have much luck.. especialy if two seem stripped down and one seems fairly complete. Also check out the seller.. talk to him.. find out where he got them or why by asking if he knows about there history.. if there isn't history then you know whats up.
But heck like the others said.. if you cared enough to stop and then even more to write us you should probly get them.