Good deal?

If there aren't any rust issues other than the floors then it's a good deal for someone who has the money.

Since you would have to sell your current DD to get it, then it is a definite no for me. You already know your truck runs well enough to be a DD. You have no idea whats going on with this Dart. He said one of the brake lines broke while bleeding. That's probably just the tip of the iceberg. You could be talking a few thousand dollars just to get it as reliable as your current vehicle. After adding up what it may need (suspension kit, brakes, tires, all new fluids etc) it may not be worth it on your budget. Slant sixes will run all day but who knows what parts were put on it? Could have a crappy remanufactured carb and distributor. Could have leaky manifold gaskets. You just never know.

I say be patient and save up your money so you can buy a project like this without having to sell your reliable vehicle.