what should I look for?
The same things you'd look for in any 8-9 year old vehicle. Get a good clear photo of the inside of the glove box door RPO codes. That will tell you much more about what you're buying than the seller is.
He has 31" tires on the truck, will that fit stock, or does he have a small lift?
The stock wheel/tire doesn't "fit" stock. So, any increase is bound to rub. This is more a question of how much not if.
I have heard of "cranking" the torsion bars to acheive this?
Tightening torsion bar will temporarily raise the front end but it will settle back down after a hundred miles or so. In the mean time you will have not only raised the front end but stiffened the suspension. The stiffening doesn't go away as fast as the lift. Tightening too far creates a sharp angle where the axle shaft meets the wheel and all sorts of bad things will happen soon after.
Will these tires totally kill the gas mileage?
If you're concerned about gas mileage, you're looking at the wrong truck regardless of tires.
I don't know what the seller means by "OFFROAD PKG/TOW PKG" neither are factory options. ZR5 is an appearance package (roof rack, side step tubes, bed rails & graphics) Maybe they added something after market?
Also, it may be a rare model but the cc was in full production for at least 4 years (2001-2004) and there were even a few made in 2000.
They're asking about $3,000 too much (depending on what exactly the seller means by "LOW MLS") but maybe that's a fair price in "EASTERN OH"?
[size=67][i]"Some people are like Slinkies. Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs."[/i][/size]