Many cars to check...I would go for a Subaru 22B (elusive...), a Taevon FTO (expensive!) or the Spoon Integra fully tuned (you'll have to win it). Better than buying your Skyline, try to win one...there are many fairly good prize cars. All these cars can blast 591HP opponents.
You could also upgrade your old Supra for a new RZ model, they're easy to drive and quite fast too...
Do not neglect used cars and underdogs (Lancia Delta, Audi S4,Nissan Silvia, Renault Clio V6), you'd be surpised to know how many great used cars you can get.
So many cars so little time...maybe you could buy one of these and test it. If you are not pleased with it just reload the game.