I have raced my SVTF in Gstock for 3 years, and they are competitive in both classes. (At least at the regional level.) The Mini Cooper and Cooper S own H and G Stock respectively at the national level.
The car is extrememly fun and fast right out of the box!
Don't waste another minute waiting on parts, or anything else. Just get out there and start racing! You will suck at first, and you will be slower than many other cars, but the only thing you will regret is not starting earlier...