Yea as Andy has mentioned, they should all be run on 98 if un-tuned as they all run super aggressive timing and will ping even on 98. If it's been on 91 it will be pulling timing constantly and then trying to increase timing occasionally which means it will be constantly running into ping/pre-detonation. The good thing is it will be detecting it at low RPM and pulling timing so it might be fine. Compression test, make sure its running through the VOD/ Turbo change over fine and a good service and mechanical inspection prior to purchasing would be wise.
In regards to road tuning for lower octane, I wouldn't wast too much money on that. just save up do some breathing mods to the intake and exhaust and get a professional tune. If you need timing pulled to run low octane fuel you could have someone tickle up your map remotely. but you would still need a $200 cable to flash it on.