If the interior and exterior are near flawless, it drives to your satisfaction, and you are comfortable in it (make sure you are happy with the headroom). Make sure you give it a very thorough test drive under all normal conditions you drive under - local and highway driving. If you don't take it on the highway, you may miss nuances on how it handles at speed and may need a wheel alignment or balancing or something like that if you get some shaking at higher speeds. It should be rock solid and stable at speed.
I also recommend driving a v8 before buying a v6 if you come across one.
If all this checks out and thats the truck you like, then make it yours.
Offer $20k and end up in the middle or lower. Don't fall in love and be reticent about walking away if they don't go down to $22k or better.