In terms of a hardware oriented DIY starting point, particularly if you're new to soldering etc, I think the brain is a good start... It'll give you 16 knobs/faders and 16 buttons as well as 16 LED's. It's reconfigurable in terms of the message types it can output, and comes with a usb interface... I built a foot controller with it (all buttons, no LED's) and my review is at amazon:
http://www.amazon.com/Livid-Instruments ... ivid+brain
The larger Brain system looks pretty cool, and appears to be pretty easy to build on assuming you're willing to use Livid's "Building Blocks".
Ultimately though, bang for your buck-wise, the lemur on the ipad is the way to go in terms of sheer flexibility. If you want something larger, a 10" or larger android tablet with touchOSC is another, similar, option.
http://shop.lividinstruments.com/brain-jr/
Good luck building!
-S!