Slop Reduction:
The stock TRX hubs got sloppy very quickly, so order some RPM hubs. They have their naysayers, but I'm fully into them. I just did a full rebuild of my RPM hubs after over a year on them and they're doing great. They zero out slop and a tiny bit of graphite dust on the balls eliminates any stiction. The way the retainer cups are designed they don't get tightened to the hub body (this is a very important detail), they float and pinch the pivot balls so if there's any wear over time, you just snug the cup screws a little, squeezing them a bit closer to the hub.
In my experience, the steel hollow balls didn't take care of much slop at all - even in new rods. So I took a different approach:
Toe Floss. Also after about a year I've only had to redo two wraps. The teflon tape also minimizes wear in the rod ends so they continue stay slop-free.
Bearings for the rockers and for the steering post. Super smoothness here just makes all the suspension work better. Bushings on the steering post wear quickly, adding precarious slop. You can pick these up from Avid RC, for example, just make sure to get the right sizes and amount.
Tires:
Get tires suited to how you want to drive the car... the stock Rally set are very sketchy for grip driving and not slidey enough for drift (unless the road is wet). They're really just good enough to get yourself into trouble... I've tried a bunch of grip tires (4tec, Austar, HPI) and prefer both the D and Pro compound HPI X Pattern.
For drifting HPI T Drift or the R2 copies are my faves.
(
wheels are fun too, R2 Hobbies or HPI for these...)
Other worthwhile bits:
TRX 3374 heatsink is a nice piece, helps keep the motor cooler and matches perfectly.
Power:
SPC LiPo

(and a good charger)
Springs:
Found the stock green 2.9 springs to be too soft, so went with Traxxas 4.3 (7342, front) and 3.4 (7341, rear). Playing around with stiffer custom shocks but tend to come back to this combo.
Shocks:
The 7061X are very nice shock set (I run them now) but IMO not a critical first upgrade. The stock shocks will be fine for a while. After a few rebuilds and the shafts wear and they get leaky, then go for a full X kit.