Well if the guy said it's a V8 and it's an '84 more than likely it's the 308.
As for the squeeling and jolting. That could be a number of thing from the rotor being jacked up to a siezed caliper. The squeeling might possibly be even just a dust cover rubbing the rotor.
If the smell is that bad on the inside of the car it's probably the exhaust manifold, I had the same problem with my 86 Supreme. It's not a hard job just time consuming.
If the wiring is still there then yeah just buy the headlight and install it. If the wiring isn't there you are gonna have some fun re-wiring that.
As for buying big stuff like seats my opinion is check out junk yards in the southwestern states and see if they will ship them. You can get away alot cheaper that way than buying them brand new.
Now the key being stuck is probably gonna be your easiest repair just replace the ignition module. Most of the time when someone snaps a key off in it they turn to junk anyways so best to just replace it.
Well yeah sometimes it's a good idea to tap the accelerator to get the car started. I'm assuming being an '84 it's still carborated and not fuel injected. It's probably got the Quadrajet right? If it does the first thing you will want to check with all the other problems is the fuel filter on the front of the carb. But word to the wise be careful the threads on that carb are notorious for stripping out. So be very catious you don't cross thread it when putting a new one in.
Well I hope this helped you out a little.