Should I Replacing Entire Plumbing / Electrical?

[Eric Yu]

Hi all!

This is the first time I'm working on a rehab that's more than just cosmetic. We're finishing a basement & also considering updating electrical / plumbing. Our contractor said we should definitely replace the electric panel, but might be able to get away with using the old wiring. He said the plumbing could be okay as well. The home was built in 1940, and I think most of the stuff is original. 

My questions: 

1) How do you gauge whether or not you should replace the entire electrical / plumbing system? 

2) Generally, what's the cost for replacing the entire thing?

3) If we don't replace it now, would it need to be replaced in the future anyway?

Also, any other contractor recommendations in the North Seattle area? Trying to get some more bids as well :) 

Cheers!