My recommendation is start with maintenance. Find a ~15 year old car that is cheap for a reason and fix it up. For example, in my area F30 328i with N20/N26 are dirt cheap because everyone knows that the timing chain loves to stretch and take out the cylinder head around 80k miles. Buy one, do all the maintenance on it, if you're still motivated after that an N20/N26 can be easily modified for more power.