Nothing worth worrying about.
85-89 were pretty much the same with some 85's needing a warranty update to the transmission. They had analog ignition which was changed in 90. The electrical will swap for those early years but most everything will not swap to the later models (Coils, V-Boost, and some misc will).
90-92 were pretty much the same as the 85-89 bikes. They had the same forks, brakes, and most of the cosmetic stuff. Electrically they updated everything to digital ignition and improved the starter to a HD version (which does retro fit to the early vmax's).
93 was the first year for the bigger brakes and forks. Other then that there was no real change.
96 was the first year for a new case design. It redesigned the case, added more internal webbing, and changed to a conventional spin-on filter type.
99 had the last major trans update with a new shift segment design (which can be retro-fitted into the earlier years).
01 was the first year for the factory rock guards on the front forks.
Various minor changes happened each year with bearing updates (mostly to sealed bearings) and minor other stuff.
96 and up would be my first choice though I don't even really worry much about the year since there isn't all that much difference to me anyway. Mileage, maintenance, overall appearance, and add-ons are stuff i look for besides the obvious price.
Sean