Hi Shoppergal
This is a great idea. We created one in our own home. The P.C. station is much used and its great to have it out of the way. The only downfall is that it can get a bit cramped though, but its a small trade-off. I suggest that you have your builder install some lights, we put in recessed, on a dimmer switch and set of double powerpoints. I am against using extensions as far as possible because of the increase in resistance on these units. You need the lights to take notes and such like and it's also less strain on the eyes.
As far as the unit itself you can buy one that suits your space and requirements as in storage of hardcopy and placing of printer, scanner etc. To have one built is not cost effective and you might also wish to change your set-up in future as well. A custom built unit might prevent this if the screen, tower and acessories are of a different size to what you have now. Hope this helps you.