i'll post an idea on this one..... i recon you pick a car with awesome handling to start with.. my pick would be the Seat cupra cup R... the one that was in the DLC.... it handles like a dream and has quite a bit of power in stock form.... then set waht upgrades are allowed.... something like only gearbox, clutch, brakes, flywheel... all the handling stuff except the suspension and adjustable stuff... that way the guys who cant tune arent at a disadvantage as the only thing you can adjust will be gear ratios and brake bias...
then throw on all the body parts for aero... set they tyres and wheels which are to be used... and work out what power mods (if any) are to be used
that way everyone is in the same car... and the only setup differences would be brakes and gearbox.
i think it would be sweet