Car buying is also a very emotional, if not purely emotional decision. Bring a practical girlfriend, wrench buddy or the missus with you to help ground you.
If the deal seems too good, especially on a high milage truck; expect to spend another half of your buying budget on maintenence and wear parts straight off the bat. And ultimately?
Know when to say "Cheeze it!" and walk away from it. A local buddy of mine and I went to go look at a $4250 used Lincoln Phaeton. We had to ditch it. Despite low mileage, dealership was asking for too much for a non-awd car, and it had low oil pressure at start up for 20 seconds. Clattered like a box full of sewing machine parts tumbling down the stairs.
Despite the shine, the newness, it was neglected and ditched by its 1st owner when rolling up on its first major service interval.
The dealership wanted him to have at least a 730 credit score before financing the rest despite dropping his checkbook to the tune of $1200 down. They wanted $380/ month payments.
I said to him, "Let's go." He told the sales creature, "Get f**ked."
He's decided to spend the $$$ repairing his hoopty.