I would be concerned about the pipes. Consult a plumbing contractor and find out how much it would cost you to replumb the whole house. That is the amount you are risking. Make sure you can afford twice that amount, then decide if you want the house.

Re-plumbs of houses with lines in slabs are normally done in the attic.

Is it a reasonable attic to work in?

NJ might be too cold a climate for an attic re-plumb. It is done in the SW all the time in more moderate climates.

It is no fun having a backhoe in your living room.

Yeah--I do worry about running the plumbing through the attic because of freezing. It is an easy attic to work in, though.

Here in Texas we can have a static water leak test done to see if there is a leak in the supply lines within and under the slab. They put a pressure reading gauge on the line, shut the water off at the street, ensure all fixtures are off and sees if the lines hold pressure over a period of time. Cost approximately $200.

6.1 doesn't seem too high, to me. Around here, below 5 isn't unusual, and a house plumbed with "M" wall copper (worst case) would have had pinholes after 20 years...that place is over 40. You could ask for a section of easily accessed pipe to be autopsied. Corrosion will be obvious.

I'd be more concerned about slab construction in your location. It CAN be done well in the north, but 1970 seems like the dark ages for that.

OTOH, re-plumbing the place with slab would be fairly easy, and other concerns could be addressed at the same time.

But it isn't a hassle-free purchase.