The T25 is a JUNK turbo.. You can buy one in good condition for anywhere around 50-200. Personally i would not get one for more than $100.

If you are just looking for a cheap turbo get the mistu. 14b...Great little turbo, and can pick up for around or under $100.
I'm not sure what you can do about a manifold.

your car will not be able to support much boost without internal mods. Maybe 5-8psi just to be safe. Personally I would not run much more boost than that on a stock LS motor.

P.S. Please dont forget your fuel system!!