Donīt buy a too low millage car. They sometimes have engine problems. Do a "matching number" check. Let check the Clutch wear percentage, under 50% it is possible, that the Clutch management is inaccurate and you need a new clutch. Check if all updates are done. Have a look at the brakes.
Check the tire age! Check if the suspenson has any gaps. Check paintwork, if it is flat, it maybe was repainted.
History, general condition, ... I think, basics donīt need to be explained.
Ask the guy, You get Your F50 from. He has nomally one ore two for sale. Currently he has a black one for sale.
BUT!!!!!! If you think about buying a 288 GTO, maybe start with this car first. It is easier to get a good Enzo than a good GTO!