I would say if you are leaving the stock motor in then nothing. Things that may break and are worth having a spare of are
LA216
LA210
LA219-7
These are the only parts i have broken on my RTR, recommend some new tyres with foam inserts too as the RTR ones are poor.
Can recommend Jez on here for ZX5 spares, been very good to me.
If you are going to put a hotter motor/esc in it then i would also recommend the following as the plastic parts will fail quite quickly
2 x Steel diff halves LA201 (for front & back) about £10 per set
1 x Universal front driveshafts - 68mm LA231 about £30
1 x Rear wheel axles LA233 about £6.00
1 x Dogbone rear driveshafts LA234 abiut £6.00
You can see what all the part numbers are by using the manual
Hope it helps, had a fast learning curve when i got my RTR
