I think the larger thing to point out is the wimpy 1/10th scale motor and ESC running on 2S lipo. That will blow up in your face. You need a 1/8th scale ESC and motor on at least 3S. You are asking for problems with that setup.

To answer your question though.....for a 3906 to be brushless ready, you need:

Steel idler gear
steel center CVDs
and the newer emaxx diffs, a-arms, axles, axle carries, and turnbuckles.
You also need to lock the transmission into second gear.

It is a LOT of money to get a 3906 up to snuff. I should know, I have one.