I'd have a play with the trial versions of some mixing apps like traktor, deckadance, etc just to get a feel for different interfaces and layouts.
That should get you start thinking about what you want from hardware i.e how many faders, knobs, what you need triggers for.
Actually there's a really awesome piece of software out there that lets you turn a Wacom graphics tablet into a midi controller.
You quite literally 'draw' the knobs, faders, buttons - all the controls you need - then print out what you've designed, place it on your tablet, and voila!
A touchpad midi controller

It's far from the be all end all controllerist piece of hardware, but for someone just starting to explore what works for them having total flexibility of your layout can really help.