As others have stated you are pretty much on your own once you step off the plane. Immigration has never been a problem for me and has always moved quickly, same with customs. As for lounges ? I want to get to my Hotel ( Sheraton Grande is my Hotel of Choice) as soon as possible. It is a late arrival and I would imagine the lounges are closed anyways. I have been in the Thai Lounge many times while waiting on domestic flights and while nothing special they always had a good supply of fruits and munchies as well as drinks, and a friendly staff.
Currency exchange booths are still open so I would recommend getting some baht at the first one you see. You will need it. The rates have not been bad there on my trips.If it is your first time to Bangkok I would recommend you go to the Taxi stand to arrange for your transport! You will have plenty of people at Don Muang to welcome you to the land of smiles offering rides and hotels The taxi stand will have someone who speaks fairly good english and will write down your destination in Thai to give to the Taxi driver. Much less chance for problems in Bangkok in the middle of the night. For a few hundred more baht you can get (Limo)sedan service. Without this turning into a guide I would just like to add
1)Book at least your first night at a hotel in advance.
2)Use the Taxi stand or limo service
3)See as much of Thailand as you can
and finaly don't forget to hang on to 500 baht per person for departure tax as it is not included in your ticket. At least it has never been included on mine in all classes of service