1. What exactly are you looking for? A stock turbocharged Mazda engine or a N/A engine to be turbocharged?
If you are looking for a stock turbocharged Mazda motor, you have 2 choices for your Protege, both are labelled "BPT" however one(out of Familia GT-Xs) is 180hp, while the other(out of 323 GT-Rs) is rated at 210hp. Corksport carries them, as you mentioned, but you can find them on E-bay aswell. You'll be paying $800(for just the motor) or around $1500 for a front clip(includes motor, trans, wire harness, ECU, etc etc).
2. 6 speed? What's the purpose of that? First off, you won't find one, and the BP motor has plenty of low end power that shifting through an extra gear doesn't do much.
3. You already have a slip-differential, all it does is slip and you get 1 tire fire consistently, atleast I do.
Limited slip units have been retrofitted from the MazdaSpeed Protege to the 1st gen Protege's G-series(or F?) transmission. I, however, don't know how it's done so I can't explain further.