I have run my SC104x4 for 2 seasons now with minimal upgrades. I've replaced the suspension arms once, rebuilt the diffs a few times and played with shimming to remove some of the extra play/wobble, and upgraded to Garodiscs in the slipper. With those small extras, the truck is more than fun to drive, and has been very reliable.

If you're good at casing jumps, I would suggest a chassis brace. Beyond that, not much else is required to have fun with the truck.

To be competitve racing (which I know you mentioned, you RUN on a track, but do not RACE on a track) the centre diff is where my next upgrade money is going....

And even with that, all the upgrades I mentioned you can probably get for under $40 for having fun or $80 for racing. $17 for the Garos or $60 for the cdiff, $15 for a brace, and $5 for proper washers to use in the diff. (Don't need garodiscs if you get the Cdiff.)

Again, just my opinion.....even though I know *COUGH* SOMEONE *COUGH WC* will say you don't need that stuff.

$5 for proper shims will save wear and tear on your diff and CVDS
$15 will help protect your chassis from bending and your belt from stretching
$17 will mean that you don't have to take your slipper pads out after every battery pack and sand the glaze off of them so that they will be ok for the next run.