12v is a cast iron block.
24v is an aluminum block.
12v sound better with an exhaust in my opinion.
if over 100k check with owner / seller to insure the timing chains were replaced w/ tensioner. no way to physically check without dropping transmission on either motor.
he should have reciepts.
if it wasnt done.. youll want that done immediately.
other then that its pretty much straight forward like any other car, brakes, tires, etc.
also there is a coolant pipe that runs under the intake manifold across the front of the block "crack pipe" as its referred.. its plastic from vw and known to crack and break. if that happens.. you would wana replace with a billet crack pipe to prevent this same issue from reoccuring. but it isnt like a serious deal breaker like a broken or slapping timing chain.
make sure oil was changed regularly.
g12 coolant (pinkish orangeish color)