nimh will certainly work in the car, just be aware a ni-cad charger CANNOT charge a nimh battery. You'll need one that is nimh compatable or just need to find a ni-cad
battery. Check your charger to be sure of what it can handle. An ESC is the single best upgrade you can make for that car. For the ESC, you didn't say what motor you had in the car, no need to get anything expensive if you ...