Sorry, I don't have any advice about 240 electrical... it sounds like the alternator ground is broken or faulty. There's a grey block on the firewall with a bunch of wires going into it, check this as well. But I will say this alone is not a reason to give up. The Brickboarders are pretty smart and experienced guys, and should be able to point you in the right direction. Also, I don't know what your mechanic or the parts-guy told you, but 240s used about 5 different alternators (at least), and they don't all hook up the same. (Ask me how I know!)

If the rust is as bad as you say, though...

I don't want sound preachy, by any means, but a couple of things occur to me. First, if I was you, I would look on this as a learning experience. Every car I've owned (13 so far) has taught me something new to look out for, when considering buying a used car. Also, each car has taught me how to repair/replace something new. Other than (arrgh!) brakes, every car turned out to have a different weakness. Structure rust and rotten engine-harness wiring are not uncommon with 240s, unfortunately.

Also, again I don't wanna preach, but if this car has been a "nightmare", and believe me I sympathize more than you could know, why would you want to sell it? Why pass your nightmare on to somebody else? Strip all the useable parts, cheer when the junk-man hauls it out of your life forever, and look for a better one- As a rule, they're really great cars.

Matt