Ive ran the VXL in the 3906 Maxx and its a small improvement over stock, but not much. The 3905 Maxx is a bit longer and bigger than the older one your refering to. It would work ofcourse, but you would have to gear it so loww that it wouldnt gain much speed over stock. I would look at the Castle Mamba Monster Max 2200 combo as well. Its not "waterproof", but they are water resistant. Ive ran my E Revo brushless w/MMM in wet conditions with no issues. I wouldnt suggest running any RC under water. Even the VXL systems. If you dont have $300to drop into a brushless and thats a reason you where lookin into a VXL you could consider the XERun 80A combo. There about the same cost and suited to run 1/8s and MTs. And there water resistant. I have the ESC in my Maxx and have ran it in snow, rain, mud with no problems. Hobbywing states there waterproof on there site. Ive had my maxx's chassie packed full of snow after winter bashing and it hasnt failed yet. And to run two batteriies youll need to soilder the plugs on in a seriers curcuit to double voltage.
http://cgi.ebay.com/XERUN-2250KV-Bru...item588c541174