Need some advice ...

[Xeen]

Welcome to the board.

With that kind of budget there is a lot you can do for the old girl.
I would personally yank out that old big block and transmission and if it's numbers matching put it in a crate and store it.
I would then go out and buy the entire drivetrain out of a TBSS and retrofit it.
As far as 4 wheel disk brakes, yes, yes and yes.
The ride quality... I'm afraid there isn't a whole lot you can do for a factory leaf spring suspension system, it's going to ride like a lumber wagon no matter what IMO.
You will come in well under the 15k mark if you are smart about who you let work on it and don't let them lie to you and give you an outrageous price tag to do the upgrades.