I won't pretend to know what would be good for you, and honestly don't know where would be a possibly better resource (as my interests have always been consume oriented).
I'd make some comment about "doesn't matter as long as it gets you dancing like this" and then link some ridiculous scene from Killer Workout or Perfect (the Jamie Lee Curtis movie), but I don't wanna be too much of a dbag.
Does it absolutely need an iPod dock or could you transfer that to newer hardware (iPod Touch/iPhone/iPad/other Apple stuff)?
Do you have any special needs for any other equipment (do you need multi-disc CD changer for instance)? I'm assuming the CD player is so each instructor can utilize their own preferred music (which is likely tailored to their class/workout routine)? Would letting them be able to plug in or wirelessly utilize their phones be beneficial? What about having the ability to playback video (would it be something you'd find useful)?
What's the general layout of the studio?
What could possibly go wrong with all the armchair professional audiophiles giving you advice????????????????????????????????????????????????????????????
There's people who legit do professional audio stuff on here. Rubicon for instance is a full on bad@$$ (she runs the stage equipment for a large cruise ship and its entertainment lounge area if I remember right, and while that might not sound terribly technical, she's far more knowledgeable than that; not sure if she's worked in the recording industry but she knows a lot of stuff about that aspect and technical stuff that blows my mind almost everytime she talks about audio and electrical related technical stuff).