What should I be carful of so as not to get ripped off, and get a car that will fall apart/break down on me.

 

The first thing that I'd check out would be if the car was a V8 model, then all the wiring should fit.
next thing on your list would be check out what kind of tranny you got under the car, the car you said is an 84 it'll most likely have a 4spd. automatic or the weakly 5spd. standard.
The tranny for a 396 would be the turbo 400 for big HP or the turbo 350 for less HP, my brother had a 73 with a 454 about 400HP and a plain turbo 350 it lasted for years.
Then check the rear end yoyu might need a little steeper gear, maybe a 3.73 or even a4.11.
Then a set of frame connectors would help a great deal.
I'd use some stock exhaust manifolds on it cause, headers will heat soak the starter.
lastly check the motor out compression test, spark plugs, wires, distributer and general engine running.
Send me a message.
Vance