[Airjer_]

I admire you preventative maintenance thoughts on fixing before there is a problem. These lines can last a long, long time. Most cars will never rust through a line. From a do it your self point of view these lines are really easy to replace. It doesn't have to look pretty it just has to work when your done.

Make sure the bleeder screws open on the wheel cylinders and the calipers before you start.

To replace the rear lines your looking at 1-2 hours, 12 to 16 feet of brake line, and some misc. fittings. Based on your areas labor rate you should be able to figure out if the shop quote seems fair. IMO it sounds pretty good.