I've heard the model 17 is a great unit- all the bugs of early drums are worked out. Actually, there are lots of self feeding drums on the market, many are fairly new models. Examples: Vermeer 1000, 1400, Bandit 1890 Intimidator, Morbark, Woodsman. I was looking for a good inexpensive used chipper, and there were more discs than drums available, so I got a Bandit 250 XP, Perkins 115 horse, live hydraulics, winch. New ~$31K, i got a 97 for 17K w/1100 hours, back in Dec 00.
One thing to check with a used chipper is bearing noise. They should be quiet, it wasn't, I had to replace them, total cost only ~$600.
If you don't have a diesel truck or a big block over 400 ci engine, i'd suggest going with a chipper no heavier than my 5800 lb unit. Morbarks, for instance are heavier, probably cause they are better built than Bandit. I've had no problems with mine, and foresee none of the metal fatigue problems I've heard about.