You could certainly build it as a 3906 and save money. The only bad point is the tranny, but you've already addressed the idler issue. The next weak point in that tranny is the roll pin that holds the second gear (small top gear) on the input shaft snaps. Replacing the roll pin with an allen wrench or drill bit of the appropriate size, either pressed in or held in with green loctite fixes it. With brushless it is recommended to put it in second gear and leave it there. First just heats up the electronics from not having enough load, and there is enough torque you don't need lower gearing.
A robinson slipper isn't necessary. Just lock the stock slipper tight.
Any original MonsterGT spur gear is a direct gear swap for the 3906 slipper.
http://www.ebay.com/itm/Associated-S...#ht_3330wt_907 The benefit to using this as a spur gear is that you step up from 32p gears to Mod1 gearing. The bigger teeth are obviously stronger.
I've changed my mind, build up that 3906!!!!!
And post pics!
