Just be aware that the waters around towns and wherever there is a river is pretty, murky. Not very inviting. Cherating was the sole exception that we noticed.

FWIW, the KUL => JHB rental for a Proton Saga or similar is MYR 1,200-1,400 all-in. If you are coming in from Singapore, you could try Mayflower which rental location is at or very close to the (original) causeway.

Batu caves is on the NW side of KL IIRC (yeah, did drive there on a previous visit en route to Cameron Highlands).