If you are going to spend that sort of money on a car that you want to keep for ever, then I would re-build the standard engine to as new spec. Ultimate performance would really not matter to me as I would not want to drive it very fast anyway.
Mine is about as original as they come and I intend to keep it that way. It is the car that I wanted 20 years ago. Now I've got one if I moddified it, it wouldn't be that car any more.