I hope you meant 12,000 miles :)
Sounds fair but keep in mind that you can often get a brand new one at 0% financing, if you're going to finance this one your monthly payments might be very close to the price of a new truck.
The only thing I've heard of is the piston slap referred to earlier, doesn't cause any damage it's just annoying at cold startups. If the truck has the 5.3 v8 it will surprise you with how good it is on fuel.
I was talking to a guy with a '99 that has the 5.3, he's at 200,000 miles and hasn't had any problems at all. Good maintenance goes a long way.