I think the easiest way would be for you to go back to KLIA. Transnasional have buses KLIA2 to Melaka.
Sepang is a small town. I don't think there are buses to Melaka from there or Sungai Pelek (which is nearer to the Avani). The local bus company Sepang Omnibus might have local buses to Port Dickson or Seremban at most.
Thanks. This is very helpful