The J lounge is normally quiet after the last QR flight departs at 3.30am or so. Even before that it is already quiet enough.
I recall a shared quiet room (with lounge chairs) at the back of the J lounge near the bathrooms. Otherwise, there are quieter corners to be had. Biggest problem is they announce every flight over and over again! But this shouldn't be an issue that late in the night.

The sama-sama express hotel is airside, while sama-sama KLIA hotel will require immigration clearance and is pricier. There is a hilton and le meridien at sentral but no st Regis yet. That's a 28 minute train ride away and the trains don't operate 24hrs.