Real nice tractors. I prefer the gas over the diesel and the row-crop over the utility. If it has a torque amplifier make sure it works. Shift through all the gears to make sure the shifter isn't overly worn out. If you can, ask to see the power take-off run something. Check to make sure the rear end oil isn't milky, it should be clear brown. Look for welds and signs of abuse. And make sure it has a good seat, because that's where your butt is going to be every time you drive it!
One of my uncles (who passed away, yesterday, after a long bout with cancer) had a 656 gas, tricycle front-end row crop tractor. He sold it to my grandfather many years ago. Grandpa put a wide front end on it... but it didn't get a lot of use. The reason that I was given, when asked why it wasn't used, is that it was thirsty. From my understanding... the 6-cylinder engine in that tractor was almost "too big" for the tractor... and it used a lot of gas. Of course, grandpa's 'big' tractor (806 diesel) wasn't really sparing with the fuel, but... I think that he'd gotten used to the fuel consumption of a diesel.
I always did like the tractor... it's still sitting in one of the machine sheds on grandpa's farm.
Aboput 15 years ago we bought a 656 gas with a wide front end on it, and a "homemade" cab on it. We use it for blowing snow all winter, and it's been very dependable. Plus, it's big enough to pull a lot of stuff (including stuck vehicles, larger trailers/wagons, etc.).
Steve is right. We go through about 5 gallons of gas for every hour of snow-blowing.