Check all the bushings on the front end. It's not a deal breaker if they need replaced, but it is a negotiation factor. Mine were good, so I couldn't tell you cost to replace.
Probably the biggest thing to look for is oil in the valley. It won't tell you how well it's been maintained, but it will tell you that you're leaking oil (likely from the HPOP), and you're going to need to do some work to it. Check what air filter he has. If it's a K&N or other "high flow" filter, pull the tube and look at the turbo vanes. Again, not a deal breaker, but you're looking at a minimal of a few hundred for a new wheel or $1K+ for a new turbo if the vanes are ate up.
Those are the quick and easy things to check out. If the owner doesn't want you pulling the intake tube to look at the turbo, bid him farewell. He'll change his tune, or you'll have saved yourself a headache.
*ETA, I was in your same shoes last May. This forum helped me out considerably, and my knowledge base has done nothing but improve from here (and elsewhere of course, but the info found here is either sound, or corrected to be sound).