The only serious problem I've had was the gearbox (but i think I was unlucky rather than it being a very common fault)
Get up to a sensible speed (50-ish), put it in 6th and give it a gentle bit of acceleration. If you hear a raspy metallic sound it may be the gearbox bearings, so walk away.
You can hear the noise in 5th too, but it's not as pronounced.