Phil, All the boards you listed are capable of midi... But the 550 also has a real nice sequencer for you to play around with... I think you need to get your hands on a board and explore it for yourself... All those midi files you can download on web sites... They all sound pretty crappy coming out of your PC speakers.. But once you copy'em to a floppy and play them through your board your going to understand a little more what we're taking about... Plus you need to learn styles and all those types of functions that will really give you an idea of what an arranger board can do...
Pro tools has a free trail you can mess with that a bit.. It's cheap... You can also try out sonar the more high dallar stuff as well for free... Don't worry... I was in your shoes once and I eventually figured it out... God luck.. It's real exciting in the begining... Just make sure you make the right choices... Especially if you don't have the wallet to make the wrong ones...
This is what you'll need in the end.
Keyboard.
Mixer.
Mic.
Software
headphones & speakers...
and don't forget the cables... They ain't cheap either... Then your good to go...
DJ