The best way to get a description of the duties and expectations of a flight nurse is to look at open position ads. Look in the job section of FlightWeb - For Air Medical Professionals
or on individual air medical program websites.
I would recommend getting at least a couple of years of ICU nursing if you decide to go the ER route, especially if you want to work in a RN/PM configuration. It's not often required to have ICU over ER experience but it is a huge advantage and you will find the transition into flight nursing less daunting. Some pediatric experience is also beneficial.
You will also need all the alphabet courses but I would not recommend getting transport or flight certifications until you are actually in a transport/flight position. Concentrate on the other courses and instructor certs are an advantage as well. By alphabet soup courses, I mean ACLS, PALS, NRP, PHTLS, ITLS, TNCC, etc. Certification in CCRN or similar is preferred and sometimes required by some flight programs.
Some hospital based flight programs work 12 hour shifts but the majority of the programs do 24's. If you do 24's, you usually average 2 shifts a week so it gives you time to do a hospital or other second job to supplement the generally lower wage you make as FN compared to a hospital nurse. Hospital programs generally pay better than community programs on average.
If you are really serious about it as a career then I would recommend you contact your local air medical team and see if you can do a ride along. You can bombard them with questions all day long and get a feel for what they really do. It is also good to know if you have trouble with air sickness before you focus on a flying career!