The only thing that change on the Banshee over the years was:
87-89 J arm frames
89-present A arm frames
wiring changes from switching voltage to component / switching voltage to ground
gearing changes 13/42 (87), 13/40 (88), 14/41 (89-xx)
I think that other then this not much has changed. In buying anything used you should look at everything unless your intent is to rebuild the entire thing. Ask about the bore size. I would make it sound like I was looking for something with a big bore so the person selling it wouldn't be afraid to be honest about how big it really is. Standard bore size is 64mm. 0.010" is equal to 0.25mm and I wouldn't bore over 0.080" although I think they make pistons for 0.100" over. Bore is the life of your motor. Bigger isn't better. Check frame structure, bearings, overall conditions. Appearance is usually (not always) a sign of maintenance also.
40 hours on a rebuild? Wow, I spend that much each trip out! With proper maintenance, the right fuel, and good quality lubricants I think you could easily get 500 hours out of a top end, like the other guy said.
SP