I've got an MST too (actually, I got 2. Bought a second hand one and a kit)
I was hoping that I could just leave it in stock form, since it's supposedly a high end kit. But the stock diff rings don't last very long, people have been using Associated diff rings (which aren't direct fit, some dremeling needed).
The plastic front end is like a fuse during hard collisions (they break), so the MST I got second hand has been reinforced by alum plate at the front. The previous owner also had some spare front arms and shock towers, as well as a new front end. These must be the parts which tend to break.
Brake disc is on the same shaft as the idler gear, so it takes a lot of punishment when you hit the brake while the engine clutch is still engaged (and the slipper clutch is set too tight) The stock plastic one would strip in such cases, and many racers use brass idler gear. My stock one is still ok though.
Alum rear uprights and wheel hexes are included in newer kits, although I don't see them wear out like some racers found =/
Racers also drill the shock piston holes to 1.6mm diameter and use around 30wt oil, but I just use 10-20wt oil and keep the original hole size.
For the wheels, the stock ones are good and cheap too. But you can use XXX-T front wheels (same offset) and Traxxas nitro truck rear wheels (makes the truck 5mm wider on each side)
For the body, there's no choice but the stock Crowd Pleazer... would be nice if Proline makes CP 2.0 for the MST.
The stock MT-12 engine comes with slide carb (normally for onroad use), most people would replace it with an OS 10ER rotary carb and Lunsford carb adapter. The manual says that it requires an engine with std shaft, but you'll have to cut it to fit.
I still haven't found the suspension setting that works for me, but I'm sure it's a very competitive truck.... if i could get mine set up. lol...
I'm just not too happy about having no choice of clutchbell/spur sizes.
Shops that sell parts for the MST:
http://www.carolinasrc.com/webstore/Scripts/default.asp
http://www.pro-s-futaba.co.jp/