I've flown American a lot to/through DFW in recent years. Yes, you should be able to select seats when you book. Some will be free, some you have to pay extra for - and I will say, I have seen AA on the seat maps block out a lot of the "better" free seats, for what that's worth - although they apparently are not the only airline that does that. If they haven't raised prices since last summer, checked bags are $25 each way for the first bag per passenger, and you can pay it at the airport (or RAC at WDW)..
The big thing to consider: look at the flights you're considering. Are they on American, itself, or on American Eagle, the commuter subsidiary/affiliate (whatever you want to call it)? When you look at the flights, the American Eagle flights should say something like "Operated by Envoy Airlines" (or Mesa or Express Jet, or whatever others they use). I've always been satisfied with American proper (no better or worse, I'm guessing, than other "legacy" airlines), but American Eagle is a MESS!!!! (Here i go on my rant again...) Nowadays,the only American flights out of my home airport are American Eagles, and I had more adventures last year on two different vacations going through DFW. I swear, it is a string of delay, delay, cancel, delay... And the Am Eagles at DFW go out of Terminal B, in a constant game of "Musical Gates" as the (probably delayed) flight you are waiting for will switch gates multiple times before the plane actually shows up and is ready to board. Plus, if you're stuck at gate B 48/49 at the end of the terminal, get ready to sit on the floor because there's just not enough seating nearby. That is the single most important reason I'm flying Delta to WDW this year: I can stay on proper full-fledged "big" Delta-operated jets the whole way.
So, in short, American: OK. American Eagle:
