PetRock

This reply is a bit late, but:

Short-term - You'll want extra driveshafts since they seem a bit soft, and after mashing your front tires into a few poles they'll bend (BS903-019 - buy two sets). You should also grab a set of screws (BS903-073 and BS903-075), and a spare set of plastic front knuckes (either the bottom screw holes in them will get too loose to keep a screw in, or a crash will strip them out - BS903-015). You'll also want a replacement for the stock brass pinion since the teeth on it will probably be worn to sharp edges in about 10 packs: Traxxas steel pinions are cheap and plentiful (TRA3943).

Longer term - You'll want replacement drive cups for the center diff since they seem to wear much faster than the cups on the front/rear diffs that go out to the tires (BS903-083) - they're aren't critical as you can drive with them really worn, but your truck will start to make a clicking noise when it moves. I would also order a set of inner hub bearings (BS903-016), since unless you take really good care of them they'll eventually seize up from being doused with dirt and water.

RedCat Upgrades - The metal steering knuckes and new-style hubs (larger 4mm screws) are worth it to never have to worry about knuckle screws ever falling out again (metal parts means you can threadlock them - BS903-111 and BS903-017N) - but don't buy the blue aluminum version (MPO-03) since they're too pricey. HobbyKing sells these parts cheap, but they're often out-of-stock. If you break your steering rack you should also consider the metal version as an upgrade (BS803-010).

Other Upgrades - Standard 12mm wheels fit: if you want to use Traxxas-style 2.8" rubber buy the Stampede-2wd-front / Stampede-4x4-all sized rims, as Stampede 2wd rear rims won't be wide enough to clear the steering linkages on the front of the Caldera. Cheap 540/550-sized Tacon motors from HobbyPartz will drop right in, and larger ESCs can be sourced cheap from HobbyKing. Traxxas Jato/E-Revo springs will fit on the Caldera shocks which will let you get the most value out of your stock shocks... but if you want a shock upgrade don't buy the RedCat aluminum upgrades (BS701-004A) since they're not a good value. Instead buy Traxxas XXL Ultra Shocks (plastic body). They're cheap (check Ebay), lots of places make springs that fit (Traxxas/Losi/Integy) and you can make them stronger if they break (metal shock caps or TiNi shafts). Finally, buy a new servo: Savox makes great bang-for-your buck fast + durable models that drop right in.

That's all I can think of for now.