Try going to KBB.com and entering the options, etc, to see what the retail values show to be. However, these cars being what they are, usually draw a good bit higher than any of the value ranking services.
I would make sure the astro roof is sound, in that, I've known several people who've had nothing but trouble with em'. Prone to leaking, and often getting off their guide tracks,,, especially when you start launching the car hard and tweaking the body. Just give it a thorough lookover, and like the others mentioned, make sure the bottom of the doors and trunk decklid are not rusting. Check them with the doors open, and feel all around the door skins down low. If it all checks out, then 6k should not be unreasonable. Just tell your buddy that in all likelihood, he will be building a motor soon after, if he plans on running it hard. 160,000 boosted miles is alot on the drivetrain, and if it isn't busted now, it's probably on its last leg if pushed too hard. At the very minimum, it would need valve springs and timing chain replaced, to prolong its life. All this is my opinion though...