Yes, but he happened to find protocol droid pieces.

So? Make them into something else. I'm sure he could find other pieces too with all the junk lying around.

If he didn't find that a protocol droid could the job, he would never had started to (re)build him.

Well, that sounds like a circular argument to me.

But the current scenario has nothing wrong with it. As far as R2's concerned.

But it does as far as Threepio's concernced. Swapping them round would make more sense, so it would have been better.