well, for me there is not much that's a deal breaker(head gasket,rare. auto box maybe) but i guess that depends how mechanical you are?
But plenty, if you spot them, you could use to knock him down.
Oil leak from cam covers, pull a plug lead for oil in plug wells and rub a hand down the side of each cam cover to head above the exhaust manifold, if its wet with oil that's a job to sort out.4hours? labour plus possible plugs/leads/dis pac...
coolant leaks, look under the car if you can, where the gear box meats the engine,2 torx bolts 4 inches apart, any signs of the cars coolant( note the colour in the bottle) or oil for that matter, dripping or staining round that area. take a torch with you and peer in under the plenum, as best you can, by/next to the egr and between the plastic inlet air pipes and again look for signs of wetness or staining/ coloured lime scale looking stuff in the v of the engine. 5 hours? labour for oil cooler cover/ stat housing/ coolant bridge
check the cam/ aux belts have been done/ when next due, 40k miles or 4 years, 400 ish at garage.
check the car is cold before starting and listen for knocking valve lifters, sounds bad but oil change or 2 usually sorts it. and make sure the exhaust manifolds are not blowing, this can disappear when it all warms up but will get worse and is a real pain to sort.
make sure the ac pump( 4 knobs bottom right pulley on aux belt) is spinning when the ac is on. the pump as about 400 new i believe, breakers on here can help for fair money tho.
oil cooler failure, oil/ sludge in the coolant
hg failure, coolant in the oil, check dip stick for mayo grey brown gravy. don't worry about mayo on the filler cap for the v6 filler neck,normal
basically make sure everything works and does not leak, but all can be fixed so don't let 1 or 2 of the above put you off, any more than that depends how much work you want to take on/ how much you can knock him down.
You never know, if it has all of the above you might get the car for free

ps check the dif is dry.
pps do paperclip test iff he'll let you
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