Never own one but always wanted.I guess you should be asking yourself first why do i need it? If you want to swap to V8 these guys will help you: http://www.renegadehybrids.com if you want to keep original engine and get more performance out of it ,then these guys will help you:http://www.devek.net .
Here is few advises which i found out in Excellence magazine: the clutch used in early 928s ( until 1987) was a multiple-disc unit and was prone to clutch chatter if abused at all.Because the 928is highly complex it can be expencive to repair.As such, it's important to buy a good example with good maintanance records.Before buying 928,have someone who really understands this particular Porsche model give it a through examination.You don't want to buy a bad one! Also was recommended that you have the bodywork checked by a reputable body shop that specializes in Porsche repair and reconstraction.Check the steering rack and ends at the bellows.If they are leaking, the rack will require replacement or repair.Make sure that the engine coolant has been changed every two or three years, as it must have an active anti-corrosion agent to prevent corrosion of the alloy block and head.Corrosion can ruin one of these engines by eroding the sealing surface between the block and cylinder head.Another problem that some 928s will have is noisy torque tube bearings.Be critical of drivetrain noices in any potential purchase.The cam-drive belts are crucial maintenance items in the later four valve cars as well as the two Euro cars.If a belt fails, it can cause expencive damage to the valve gear and possibly to the engine internals.Be sure that the 928 you are considering has regular belt maintenance every 30-45000 miles, which should be the regular replacement interval.It's important to inspect the belt every time the car is serviced.
Hope this helps.Happy New Year !!!!