I would just go stage 2, same installation requirements and you will be happy doing it once.
Paying off 1/3 of your debt will save you some interest maybe, but the difference you save may not bring as much to the table 500+ WHP will.
I made the same decision a while back, if you turn around and don't want the car simply remove and recover your money and it will be a small amount paid to play. The only thing I can say is that once you drive it S/C you will not be entertained in a stock or FBO M3. The thrill you get hitting boost is something out of this world.
Hope you make the right decision (and go S/C!
) and don't look back, you will not regret it that's for sure!