Sounds like the mechanic wanted to pick up a cheap jeep. Brake lines aren't that hard or expensive to replace. If you're in the NE, a more likely reason for the lareg amount of rust is the liquid ice melter they spray on the roads in winter. That stuff is extremely corrossive. I had a brake line blow out from rust. Fortunately, it happened at start up, so there was no fire and no careening off a cliff or into a tree.
If you DIY it, make sure you use a tubing bender to bend the lines or they'll kink. I imagine a shop would do the whole job for a few hundred bucks. From what you wrote, there's absolutely no reason to junk the rig. Just fix or replace things as they require it, and it will keep going for a long time. It might cost a few hundred bucks and some time here and there, but it will be much less than a car payment, and you'll get to know your jeep much better.