dead engine
 

My engine went 2 weeks ago; so 'm looking for an s5 turbo 2 engine or buy a very clean 84 gls 12a streetported from my friend for $3k

So are you asking which to get? Cause it will be a lot easier just to replace with what was in it and get the t2 engine, unless your car was n/a, then you should get the whole drivetrain. There is a lot of info missing here to help answer what i think is your question. What year is the car, what model, ect.
If your car is s5 i would just get another s5 t2 motor, if your car is s4 stick with looking for s4 motors, rewiring is a headache. You can find a lot of t2 motors for cheaper than your friends 12a, plus you wont have to rewire anything, and i think you will be happy with the power difference. The 12amight be pretty decent in a first gen but the second is a little heavier car.