You've had some good advice so far... is it the 140 perkins or the heavy duty version? come with a winch or a grapple maybe?
Take a look all over the frame, don't be too worried about cracks, it's part of every day life with a chipper that size but use them as a bargaining chip. If it looks nice and clean and well looked after then well worth the money, but if it looks hammered, faded paint, peeling stickers, lots of poor looking welds, then don't be afraid to walk away. Lots of people are selling off big chippers to buy smaller ones! Maintenance/repair costs are very high on machine this size, and they have more problems than the older machines do and are harder to trouble shoot. A buddy with an 1890 went through all sorts of hell last year trying to work out why it wouldn't hold revs up high enough for autofeed to work. Went through $$$$ with injector pumps, injectors, computer, sensors, tank cleaned out etc etc... seems hard to find a guy who can get their head around accurate trouble shooting on these machines. Dealer support is critical.
If it has the winch, take a good look at the feed wheels. Some guys are real hard on the winch and haul medium trees hard up against the wheels, lifting the tree right up off the ground 6'-8' in the air, then quickly slam the chute open. The 'teeth' on the wheel end up bent and cracked, and the bearings end up shot too.
You'll be horrified at the cost of running a machine this size, but they sure can make some money (if you've got the work!)