I'm using a second account to play the game with zero credits. I'm at over 70% completion without spending a single credit apart from the mandatory purchase of the Honda Fitt. I have plenty of cars due to winning them which in turn has helped me win every race so far. And with the help of the 200% log in bonus and some large seasonal payouts I'm on well over 20,000,000 credits and yet to even complete all the Redbull events.
As for the original question, I have to say why? A whole new game is available to play where just for taking part in seasonal events and easy Redbull events you get some awesome free gifts. PD need to put all their focus in to GT6 and just forget about GT5. It makes me so angry that each week without fail they keep updating GT5 with seasonals yet their flagship game seems largely ignored and the seasonals we do get are hot laps, not races! Most of GT6 is very very good but it could be brilliant if they put all their focus into it. Personally I cant wait for them to switch off the online in GT5 as then they have no excuse to not fully commit to 6.