N/A definitely. And this is why;
1) FI will bump you right into Street Mod at least. From there, you have a choice to whether spend another $20K or so to be somewhat competitive in your region, or spend far less and not be competitive anywhere. You'll be competing against cars with a lot less weight, a LOT more horse, far more grip, etc. etc. I honestly don't think an SM Focus could make it beyond a local level.
2) Less money on the motor is more money on the suspension, where it really matters in AutoX.
3) N/A engines are the easiest to modulate and control on track, giving you more precision, which makes a HUGE difference on course.
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...