I had a next door neighbor who renovated organs once, he had an amazing midi controller made from the actual keyboard and mechanism of a knackered old piano. I seem to remember that he used a kit (about £20) from Maplins to do all the scanning and midi bits. He had another one using the keyboard from an old lowry organ too, come to think of it.

If you go for off-the-shelf stuff, the best piano sound is gonna come from a module anyway. A 1/2u Alesis nano-piano only costs £80 for what is a fantastic sound. That way you can use any controller - a 4 octave one comes in at about £50, a five octave one a bit more. The good thing about this is its really really light and transportable. You'd just need to build a box/stand for it yourself.

I uses an eight octave controller (a lot of keys, iknow, but its for jazz stuff) keyboard and Yamaha piano module. It all fits in a soft case and I can carry it on the train without too much trouble.