Keyboard master is just a title the forum gives you when you post a lot
For a controller, I'd probably recommend the M-Audio Axiom 25. I've used the 61-key model before and it's great. There are a few others out there on the market, but other than the feel of the keys (try them to see what you like), there's not much else that really matters. Maybe a lot of knobs, buttons, or sliders, so that you have more control over things on the rack module. I'd basically program a controller so it always sent to the rack on a constant MIDI channel and so all the buttons and knobs always sent out specific MIDI CC messages, and let the module figure out which sounds to play.
For a module, you're basically looking at the rack mount equivalent of one of the standard pro-level workstation keyboards: the Korg M3-M, Yamaha Motif XS Rack, or the Roland Fantom-XR (they don't have a rack version of their Fantom G, their current top-of-the-line workstation). There are, of course, rack mount models of some of their older products as well, which you'd be more likely to find more cheaply.
I don't know that I'd recommend this route though. Especially if you're looking for used gear, you can probably get a workstation keyboard, especially one of the lower end models (Korg M50 or TR, Yamaha MO or MM, Roland Juno) for about the same price as a module and controller, and the workstation keyboard will have more keys (25 is really cramped) and the keyboard will work much better with the sound unit since they're all one piece.