Castle ESC's are not very good for racing. If you get a race grade servo which pulls lots of power, chances are you will run into glitches and may have to purchase an additional external BEC. Most other brands of ESC's are rock solid to include HobbyWing XeRun.
Most racers prefer a sensored motor to get a little more throttle control which gives better traction in tight corners.
+1 to the TenShock combo here with a 4600kV motor:
If you want to save some cash and forgo the sensored setup, then this is a pretty good deal here with the 4400kV motor:
Believe it or not, you will get faster lap times with a better servo than a better motor.
A decent mid grade club level servo would be the Savox 1258TG, if you want to go high end, then check out the RadioPost servos which offer 32 bit precision and are blazing fast
Club Racer: SCT + eBuggy + GT8e + GT18e + USGT
Basher: Monster Trucks + Crawlers + Drifters + Motorcycles