A/C swaps "usually" occur just for mx reasons. If you have multiple a/c in the same base you could fly a different one every day. If you get outbased somewhere remote, you might have the same a/c for a month or more at a time. Get to know the a/c pretty well, you might find one you like more than others and you could possibly request it most of the time. My favorite Baron was always 9189Q...not the best looking, but it was one of the lightest and it would perform like a champ...had the vg's on it as well which always made you feel a little safer at slower speeds. Good GPS and a good radar...she never let me down. It always seems that the best looking a/c with the most goodies seemed to have the most problems, so less could be more.
As far as a good base...stay away from the NE. BHM seems like a good base, really cheap to live there, and lots of stuff to do...if you are into the outdoors. BHM offers the opportunity to move into the lear as well...at least with the charter dept that is. You wouldn't have to move to get a spot, plus they are 60's. I am sure there are others, but once you get north of the middle of the country it gets too cold for me, so i do not know much about it. Good luck, and bid somwhere you will not mind staying for a year.