UPDATE: This is an old article, before iPads existed. You are better off just getting a desk and searching newer components than what is listed.
I've decided to come up with some designs, and at least a partial parts list for building your own keyboard with integrated computer and touchscreen. It seems like a good time to do it. LCD prices aren't too bad, and we now have high powered computers that can fit almost anywhere, like the mac mini and those PC mini-itx cases. I don't think you should have to pay over 2 thousand dollars for something similiar, and this would just be cool to make. The whole idea would to make something that is portable with all the controls in the same place.
LCD Planar Touchscreen $300 - This one is kind of ugly due to the white frame, but the price is right for a 15 inch. Getting a good touchscreen / LCD with enough screen space for the right price would be pretty important. This Planar one goes for $440 and could be a hell of a lot better.
A small Midi keyboard + Keypad for only $44. I'm not going to bank on the construction quality of this one, but it looks really Neko-ish. If you added a small, in-car-style 8-10 inch size touch screen, which are even cheaper than the ones I mentioned it might be really cute. Other than that you'd be best to go with a USB midi keyboard for whatever you can get on ebay. Even better if it was one of those keyboards that had the audio interface built in.
This might be the grandaddy solution for picking out your computer...
intel imac with touchscreen or ibook touchscreen. On the intel mac, you could have your choice of running both windows and Mac OS X with the latest chip architecture. If you went with an iBook, you could carefully dissassemble it and mount it inside your case with the keyboard midi controller you picked out. Having the computer, Keypad / touchscreen all at the same time would be effecient, as then you wouldn't have to worry about power supplies for each. I don't know how much that costs to add on.
And then your audio interface, if you didn't have it built in to your keyboard, probably get an Edirol self powered USB unit. That one is a little expensive at $240. You would probably want the built in audio interface type of midi controller, with knobs.
As you can see, I didn't address the problems of having multiple power supplies...but if your like me, you'd just have the bundled cords wrapped up out the back and buy a power-squid. Maybe with enough space, you could shove the power-squid up inside the keyboard box, to trick people, like you actually know how to run all of that stuff off the same voltage.
Chip Collection Version 1.
Cost Estimate: $1,150 ish
(LCD Touchscreen, Cheap Prodikeys Midi Controller, Intel mac mini, Edirol USB/MIDI Audio interface, + random case parts)
Cost Estimate: 1,900 ish? ($1,500 Apple iBook bought on ebay with modified touchscreen by Troll Systems, $300 Midi keyboard with audio interface built in, +$100 worth of random parts)
This could version be flat and rectangular looking more like an Elektron machine. The flatscreen would be attached at the far left end, with various components inside the box underneath, then having the keyboard tacked on its side. You could also have the flatscreen attached like version one above if you didn't like looking down.
Doing a keyboard like this would be an awesome project, and I bet you could do it for under 2000 dollars and have a pretty nice result. If you were able to use clear plexiglass as a box, you could even do etching across it and light it up from inside. Extra modifications would be knobs & such for midi control. Even sillier would be to add speakers. If you played on stage, it might be cool to have a mini lcds attached to your keyboard, facing the crowd, so you could put some kind of weird video on it. Also, no, this keyboard isn't the solution I was talking about for my previous article.