IIRC there was a recall on some of the 255s to have an ECU remap so it would be worth checking with a dealer to see if the one youre looking at is affected, and whether the work has been done.
Other than that (speaking as a mk1 v6 owner):
- check for misfires & check that it pulls well in all gears, as coils are susceptible to failing and Ive had problems with catalyst breakdown on mine
- check that all the lights come on at startup, and then go off one by one
- check for wheel damage as the suspension setup is easily damaged if a wheel has collided with a kerb
- check for cheapo tyres fitted - ones with soft sidewalls will affect handling, though of course you can easily replace with better tyres
- check carefully for damage to any of the composite panels as they are expensive to replace, though are not to expensive to repair if theres any cosmetic damage
There was a buyers guide to both the Mk1 & Mk2 in Evo mag a few months ago - might be worth trying to obtain a back-copy issue.
Other than that, I think youll be fine. I hear that the mk2 has fixes for a lot of the problems found on the mk1s, and youll certainly not regret buying one!
Cheers,
Dave