I will be arriving at DEL at around 01:00am and will have to wait for the rest of my party to arrive until 03:30am (first time in india, so cultural shock expected). Where would be the best location to do so? I will not have an ongoing connection, so any Airline Lounge should not be accessible, right? Any recommendations for a decent location?

I read about staying in the luggage carrousel area. Can any expert second this recommendation? Is there an ATM there and any possibility to get bottled water or something else to drink?

All arriving passengers in DEL are directly led to the exit. You cannot access any airside lounges unless you are transiting. Once you get thru immigration at DEL, you turn left and land at the caroussel. Once you pick up your luggage, you have to walk thru customs. They no longer check all luggage, so you can just stroll by. Before you go out and get hit on by the usual gang standing outside an international airport, there is a lounge for international arrivals to your left. This is where people wait for transfer to the domestic airport. You should be able to wait there. If you want to get something to drink, you proceed out of the passenger area (you can't return!) and you are in the general waiting area where people wait for relatives to arrive. This area is only accessible for non passengers after you pay 20-30 rupees, so it is relatively empty (in indian terms!) Here you should be able to locate a shop that sells you something (most probably tea is available.

I suggest you carry a water bottle with you that you fill in the plane, pass immigration and customs, then wait right after customs in the transfer lounge. If you are a first timer, time will fly just observing the locals around you! ;)

.... the general waiting area where people wait for relatives to arrive. This area is only accessible for non passengers after you pay 20-30 rupees, so it is relatively empty (in indian terms!) Here you should be able to locate a shop that sells you something (most probably tea is available.

It's up to 55 rupees for entrance for those waiting for someone to arrive into that area.

You might want to consider going to a nearby hotel instead. The Radisson is a 10 minute cab ride away and will be a more agreeable place to wait than the airport. I do appreciate it might make the onward logistics a bit more inconvenient though.

Thanks everyone, I think I will sit in the transfer lounge and watch...