as with all these kind of things, the bigger the engine, the more it costs to get it fixed...
ask to see as many reciepts as he has, have a real good poke around the engine bay, check everything that you can when its cold, then fire her up and watch it right until its fully warmed up... check for any knocking, rattles, andthing sus basically, leaks or signs of leaks, ask as many quetions as you can about everything.
then take it for a spin, go steady in it and see how it pulls off, torque steer or fooked running gear? any knocking again might be signs of poor maintenence...
once you come back, pop the bonnet and again check for any leaks (he may well have given the engine a wipe before you turn up) and check the gaskets...
after that give all the tyres a good kicking and if it doesnt explode, buy it.