Get hpi p/n 100905 for the tranny. It is the flux middle shaft and gear. All you need to lock your tranny in 1st, as the x and flux trannies are identical other than the middle shafts and gears. It cost me 12 bucks off ebay...
You can get the flux motor mounts complete for under 40 bucks, and the tvps around 25-30. I have not priced the battery boxes, but they can't be more than 40. Hpi uses a 20t pinion stock with a 44t spur? So if you plan to run 5s I would go with a 17-18t pinion.
http://cgi.ebay.com/HPI-Idler-Gear-4...3286.m20.l1116