1. The idler is inside the transmission housing. Inside it is the middle of the three gears. The larger is the diff, and the top gear is attached to the slipper shaft only inside the case. If you are going to use the aluminum idler I would recommend you also go with the delrin top gear. The aluminum will more easily get chewed up by the stock steel top gear. Part #'s I think are 1996X and 3195X.
2. Can't help much with gearing.... I run a 12T brushed motor geared at 18/87 I think. As far as Mamba goes the cheapest price I've seen is at Bishop Power Products http://www.b-p-p.com/products.php?cat=40
. I don't know anything about the place but prices look pretty good with free shipping.