12k mile oil changes? Eww.
I'd either inspect the car yourself, or take it to a trust worthy BMW mechanic that can spot any potential issues.
Does he have all the paperwork on the car? If it is an auto, I'd be very sketch on that. I would demand paperwork on any automatic transmission. All proper fluid changes and maintenance performed at proper intervals?
240k isn't bad for an M44, as long as it has been properly maintained. With 12k mile oil changes though, I don't particularly care for that myself. I still change mine every 3k, and it is dirty by then.
I have a 4/95 318i, and it has been practically trouble free in the 3.5 years I have owned it. A few small issues here and there, but nothing overly major, other than the differential.