Ive seen all years converted. Most recently, a 90's C4 converted to 73' RSR by Air Cooled Excellence, i think. No matter what year you choose, just get one without any rust or accidents. With the costs of the early cars still rising, I can see why you would want to spare an original. Plus, the older the car, the harder it is to find a rust free example.
If money is no object, find a 72 like Jack did. Four door RSRs are cool
when you get started, be sure and post project pics during the transformation. Good luck.