The best thing to do is to buy a car already built. There is a z car for sale in the improved touring classifieds that is a front running ITS car for 15k. It used to run in the southeast and was built by an engine builder and has just about anything you would want to do to an ITS Z. And its for sale for less than half what it would take to convert a car to ITS/vintage specs. It has a 5k dyno'd motor, RR shocks and a fully set up suspension. The car has several podium finishes. I ran against Jim Thompson in that car and it is fast and a no-excuses car. The only thing you will need is to get track time. Just my 2 cents.