MRC/Academy SB Sport Pro 1/10 4WD Buggy - Going brushless, need a little help!

[Chadworkz]

Ok, I was ordering a couple things on A-Main last night and saw that they had the MRC/Academy SB Sport Pro Carbon Limited Edition II kit (long name) on sale for $189, so I just had to buy one. I had already decided to buy a B44 soon for my new budgest brushless project, but as soon as I saw this thing at such a great price, I just had to buy it! It is a very nice buggy, with full carbon-fiber chassis, upper-deck, steering-plate, battery-strap & center driveshaft, billet aluminum motor-mount & chassis posts, front & rear ball-diffs, full CVDs, teflon-coated aluminum shocks, and lots of other great features.

Anyway, I bought a SuperSimple 50a ESC, a Turnigy 3900kV motor, and a Zippy Flightmax 2S 4000mAh 25C pack from HobbyCity (about $55 total), and this is the setup I am going to run in the SB.

I will have to wait until I get the kit so I can count the number of teeth on the pinions and spurs in the ball-diffs so that I can use the speed/gearing calculator, but I was hoping one of you might know where a good place to start would be? It comes with 3 main spur-gear sizes, one of them being 67t, but I don't know what the other 2 are...and it also comes with a 23t motor pinion, but I have many sizes.

I am not going to race, and I am not a speed freak, I just want a good bashing speed that is reliable and is capable of just enough speed to be exciting...any ideas? Thanks in advance!

Links:
The Car [here]
The ESC [here]
The Motor [here]
The Battery [here]