Q: How do you buy an old car, without getting ripped off?

Hello, I have been looking into purchasing a used 1998 Chevrolet 1500 pickup. It's a z71 package 4x4 with a 9.5 inch lift. Interior is clean with no rips in seats. The owner has the pickup posted for $6,500. However, he has also brought up some issues with the vehicle, but I just don't know how costly the fix will be. He said that the tires need to be replaced due to alignment (already aligned but needs tires) , there is a"small amount" of rust in engine bay, and that one of the third doors does not open. My biggest concern is the rust and broken third door. I guess how big of an issue is rust and the broken door compared to selling price? What should I expect to pay to have the issues resolved if I were to take it in?

My car has 126000 miles.
My car has an automatic transmission.