Having been, well, screwed, early on when I was a novice on the VPI's, here are some differences.
The VPI 16 has not been made in over 15 years. It had the vacuum tube attached to the lid and did not have a reservoir for fluid - it just was sucked into the base for evaporation.
Some 16's were upraged to 16.5's - different lid, with the vacuum tube mounted on the deck, and a plastic reservoir (later ss) mounted under the deck to hold fluid (mostly grime). In addition, some 16.5's were made that have a 16 model plate on the back. Folks at VPI explained to me that the very earliest 16.5's used the 16 name plates that were left over in stock at the time.
I don't know too many specific about the 17, only that I would like to have one and they are about 3X the price of a 16.5. A friend that I know has no problems with his and can clean many records in one sitting. I can personally attest that the 16.5 will get HOT after doing about 6-7 records. Push it too far and it WILL shut down. I am thinking of modifiying my 16.5 to include a small fan.
Sean is right, shop around for deals and, if not doing business with a verifiable dealer - check out the condition carefully before buying................