I hate dealing with KL cabbies. Unwanted extra stress. They used to be such a menace during my hops from SIN to KUL.
At 12am there will be little KL cabies around KLIA (I suspect most of them will be in the city or around Bangsar where the demand will be at that hour)
Your best bet will be the KLIA taxis (white cabs, you'll spot them easy, they have this little GPS like device on top of them)
KLIA is very near Seremban (I'd say 30-40 minutes tops, no traffic . Perhaps even less).
Going to KL Sentral if you need to get to Seremban is pointless. You'll be going in the wrong direction.