One, 30 years is NOTHING for copper pipe... there's a lot of it in use today that is WAY older than 30 years and doing great. Unless it has been connected to galvanized pipe and set up some type of corrosion, or it's been abused in some way, the 1/2 copper should be okay. Your plumber should have been able to see that kind of problem, I would think, so if it's not been mentioned, I wouldn't be too concerned. I'm not a professional plumber, but I would place my bet on copper plumbing any day.
Secondly, I don't know what it's going to cost you to replace that 2nd pressure valve on your own, and whether or not that's going to be the deal-breaker financially, but I can tell you that I would NEVER let anyone (a bank or otherwise) pressure me into making the BIGGEST PURCHASE OF MY LIFE if I'm not comfortable with the terms. Walking away is sometimes the hardest, but the wisest thing to do. There are many people writing for help on this web site who wish they would have listened that little birdie when it spoke to them at the paper-signing!
Buying a house is always a little never-wracking, but you need to trust your instincts. Are there just tons and tons of people waiting to buy this house? If so, and you've just absolutely, possitively HAVE to have this particular house, then buy it. If that's not the case, don't let the bank MAKE you do anything today. The bank isn't going to have to fund the future repairs.
That said, do not think that you will ever buy a house that needs no repair. Especially one that's 30 years old. Any house is just a big box, prone to leaks and drafts and settling, and it's filled with tons of things that have mechanicals parts that wear out and fall off and clog up. And I can almost guarantee you that something major will need repaired or replaced within the first year you own it. That's part of the "fun" of home ownership!
So, all things considered, if the rest of the structure appears to be stable, the neighborhood is great, the price is right, and you're smitten... then buy the house. If this plumbing pressure valve is that major thing you replace in the first year, then at least you've been forwarned!
I wish you many, many years of happiness in your new home.