CO27 if your staying with brushed........Advantage to the motor builders is the set up. They will center the brush hoods and maybe zap the magnets plus true the comm. Saving you some work.
I was going to try the Mystery Stock this year, but with Cleveland going all brushless I decided not to.......
Best advice is to read the forums on motor tuning (brushes and springs) and other building tips and do it yourself....You bought a lathe and truing the comm is the main thing to start with.