As manual said above^ it's an each way bet.
Pro's of buying one that hasn't been rebuilt is it can generally be rebuilt - and you will have piece of mind afterwards (if reputable shop).
Con's is it's closer to needing one (IMHO) - so have $15k spare in your account after you buy it.
Pro's of buying one that has been rebuilt (if reputable shop and receipts) - Piece of mind and ultimately cheaper purchase price
Con's - maybe a poor build. Con's - may not be able to rebuild.
Cons for both is the treatment of the vehicle.
Either way be very thorough checking the vehicle.
Finally you will generally get what you pay for either way.
Edit: Comments above re R32 GTR.