MikeyC,
I'm an NC real estate broker and a long-time DIYer. If you're a first-time home buyer (or haven't owned a home in the past 3 years), you are probably qualified for the economic stimulus tax credit of up to 10% of a first home purchase up to $8,000 if closed by Nov. 30, '09. For instance, you close on a home at $80,000 or above by then, and say you owe $3,000 in federal taxes this year...you will pay them nothing and they will send you a check for the $5,000 difference!
That should more than take care of replumbing the average house. I would go with PEX to replace the obsolete galvanized. Get 3 pro quotes in writing as already recommended, and also know the cost of all other repairs needed to bring your home up to speed.
Good Luck!
Mike