This would be my order of importance for upgrading RTR electronics:
1) Radio - Single most important upgrade, based on what's currently available today I would get the FlySky NB4, more info here (dated thread but information is relevant):
2) Servo - Some quality budget servos listed here:
3) ESC/Motor - Best to buy both at the same time
- HobbyWing is decent, but I don't recommend the stock spec for high grip tracks because the brake is weak on the ESC, it does fine for low to medium grip dirt but carpet/turf needs a stronger brake so I would recommend the XR10 Pro
- If racing stock spec, then things will get very expensive if you want to stay competitive, everyone is gonna say their favorite brand of motor is the best in the motor war. I have found it best to buy a 5% Certified motor to guarantee that I'm getting a hot motor with the strongest rotor magnet and lowest IR on the stator:
4) Charger - for stock racing you can boost performance by charging your packs at 40A, more info here:
5) Battery - Get the highest quality battery you can afford, Trinity White Carbon packs can handle 40A charge rates without breaking a sweat:
If you want to save money, stick with open mod classes, stock classes are super expensive if you want to be competitive.
Good Luck!