This is no small thing, if you aren't familiar with OOP, I'm afraid.
What you'll be wanting to look into to parse Gleed2D (which I have used and recommend) files is XML parsing [ http://www.republicofcode.com/tutorials/flash/as3xml/ ].
Level selects are essentially menus. How you'll want to execute them, however, will affect a lot of things though, so you'll have to ask something more specific. I remember however that one of @zachwlewis ' OGMO tutorials had a bit about that. (@zachwlewis, a little help on which one?)
Re. bosses, you're on your own, too broad a thing and not at all FP-related.
You can cheat the credits. Make a tall image (on GIMP or what have you) and make that the graphic of an entity. Then add that entity to the world and just move it upwards until it's gone. This'll give you a "rolling credits" style thing. If you don't need that much space, you can just skip the scrolling thing (i.e. make a credits image, make it an entity's graphic and add said entity to the world) .