I'd suggest to have a look at midi like stuv says.
With a keyboard interface you can't make yourself any fader. Then I ask myself: why all the trouble and not just buy an USB keyboard?
Next to that any keyboard interface has this problem when you try to press multiple keys. Depending on the type of hardware and what keys you press, pressing several keys at the same time may not work. Also the application you are controlling (like FreeStyler) needs to having the focus in windows for being able to receive the key presses.
If not, it doesn't work.
With midi you don't have al of this problems. And also for midi there are small ready made electronics, I believe.
Then, when I look at equipment like the Korg Nanokontrol and its really low price, I'd say: you almost can't create anything like that yourself for that amount of money.
But if you really want to (because you just want to
), I'd suggest to use the nanokontrol (this has a midi interface), open it up and use the electronics and buttons and faders for your own project.
Or just buy the nanokontrol and use that.
This is it: