I am considering reading Courant and John's famous Introduction to Calculus and Analysis as a reintroduction to, and to expand my understanding of, calculus. I was reading reviews on Amazon, and one reviewer said:
While Courant's insistence on proof does mean that the student needs to have a
basic grounding in proof methods, this is usually a standard part of the
undergraduate curriclum.
How much understanding of proofs do you think is necessary? I have read a few proofs in an introductory statistics textbook and seemed to understand them, but I don't have any sort of background in "formal proof theory," if there is such a thing.
Do you think any other books are a prerequisite and if so which ones?
You won't need much of a background. Books like that are designed to be first courses in proofs. If you want, you could use something like Richard Hammock's "The Book of Proof" to get a quick introduction to proof logic, and then head into Courant.