thanks for all advice. Still, no one answered my question - what time do i need to be at the domestic airport for a 6am flight? I've heard horror stories of security lines.

part of the desire to stay at the airport is to avoid extra hassle - finding taxi/shuttle, driving to hotel, checking-in in the middle of the night, checking out, finding another shuttle/taxi and getting to the airport. all of that for a nap of couple hours is hardly worth it. although if conditions at the airport are particularly bad and there's no clean, quiet place to sit then i guess there's no choice.
I did check Sheraton, Hyatt, and a few other hotels. They look nice but i can't justify paying $250+ for 3-4 use of the room. i doubt many FT'ers would do that if not on an expense account. We have little plans for a day after our arrival in Agra, only to sleep and rest. But we'd rather push it straight through at once than break up the trip in a few still-tiring 1-nighters.
if we stay at a hotel - it will be something that was suggessted earlier - cheap, not so nice and close.