[suicideneil]

Okay. Im not too familiar with Maxx trucks either, but I built mine from scratch so I l know every screw & nut as it were- studying the exploded views on the Traxxas website helps alot in understanding the beast.
On the diff front... red... yummy. You can get 3.3 diffs off of ebay really easily like I said & so long as you learn to start off gently, then floor it, they should hold up really well for a long time, then upgrade/replace as required, so dont be too worried about spending loads of money on stronger parts- that roller has most of the parts you need for bashing anyway.
The parts you will need though:

1. CVDs or 3.3 slider shafts & knuckles- stock emaxx shafts are poo poo with BL power.
2. Tranny, obviously. Ebay is the best bet, otherwise you need a million small parts to build one (no fun that).
3. As for the Mod1 gears, I would only go down that route if you are into serious bashing/ racing with a super powerful truck. You can get 5mm bore 32 pitch gears which will be fine.

I would though dismantle the bulkheads and clean them up with some oven cleaner to remove the worn anodzing, same too with the diff cases (dismantle first & pay attention to how they go back together). Apart from that its a pretty sound truck.