I would buy a new one (if monies was not an issue) If monies IS an issue, you have no business owning a Diablo, for it can get VERY expensive. If you STILL want a used diablo, make sure you have a dealer CHECK IT OUT for you before you buy it or put monies down on it.
I've had a 98 SV that was a true lemon. I bought it NEW, and was able after a long battle to have them give me a new car. Yet, I feel sorry for whoever ended up eventually buying that car (it went back to the distributor). My 99 Diablo Roadster, on the other hand, was a perfect car (I put over 12k miles on it in less than 8 months... / drove it every day).
If you HAVE TO buy a used one, I would check with the folks over at teamlamborghini.com (
http://www.teamlamborghini.com) / talk to Dante. Get the computer printout from the car / have the dealer explain to you and go over it with you carefully. (shows you the amount of time that the car was driven, the temperature, the RPMs, any recorded problems, etc, etc.). If the seller is unable or unwilling to provide this for you (free of charge of course), drop it and get another car. This info comes from the onboard computer behind the passenger's seat... (make sure it is the ORIGINAL one also...)
Hope this helps. Good luck!
Luigi