[MetalMan]

Here's a list of some off-the-shelf upgrades I did:

-Nitro Rustler front steering conversion with Powerline aluminum carriers and standard plastic hubs. This increases steering throw, and most importantly, allows the use of Nitro Rustler wheels which have the 5x11 bearings in the wheels (less rolling resistance).
-FLM extended front arms. After making and buying extended front arms, they broke, and these are fine so far.
-FLM aluminum bulkhead. It's just a worthwhile upgrade.
-Better steering servo and servo saver.
-For the shocks, the aluminum big bore shock caps are minimum, for front and rear. What would be even better would be all aluminum shocks.
-Aluminum idler gear.
-FLM aluminum tranny case. If you get this, the gears will all run noticably quieter. I don't have the steel/aluminum differential, and haven't stripped a plastic differential gear in a while, which I would attribute to the aluminum gearbox case.
-Rear axle carriers. I use Powerline aluminum 4-tec carriers because they are adjustable and use 5x11 bearings, but also because they work well with my dogbone conversion.
-Steel differential outputs and steel axles (they are for the 4-tec but work with the Rustler), if you are using the stock sliders. I did the dogbone conversion, and it has been working well.

One part I have left stock are the rear arms. The main reason being is that the FLM gearbox uses aluminum arm mounts, and I would probably bend the crap out of the hinge pins with aluminum arms.
Soon I'll get a thread up on my Rustler (since I finished the chassis) and it might give you some ideas.

As for a motor, if you want the ultimate speed with a motor that's readily available, use 4s Lipo with a Feiago/Wanderer 540 12s. That's the motor I have right now, and the same one that Justin runs in his extended (1/8) Rustler. If you use 5s Lipo, you're going to have to special order a motor. For 8 cells, you could use a Feigao 380C 6L, or a Feigao 380C 8t/9t.