He definitely doesn’t need any more suspension or braking parts for what he’s doing. He does need rear axles if he ever wants to launch the car at the track. They won’t last very long at all.
DSS stage 5 is 1600 bucks. I have their pro level kit but stage 5 is fine for your use. If you want to plan ahead you’d need the pro level kit and slicks to make use of the 42R.