For alternator, it is such an easy fix, I'd say use the old one. But the starter is harder, (especially if you have headers, might want to consider a new one, and the water pump, well there is a lot to do to change that, the fuel pump is a real easy fix...

It's not the last time you will wrench on it. But you don't want to find yourself a month, or a year from now saying " for another fifty bucks, I wouldn't be working on it now.