Your first investment if you want to race should be a spring kit and maybe some damper pistons and a selection of shock oil in 25wt, 30wt and 35wt.
If the car has bushings these need changing to ballraces as all parts will last you a lot longer.
The only other things you might want are cheap stuff like sticky-backed lead weights to balance the cars handling and maybe a bigger rear wing which can come in handy on some tracks if you're struggling to find grip.
Apart from that just run it and keep it clean, it'll last you longer then.
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