Answer to your first question:

Both the Korg Pa1xpro and the Motif ES only support USB 1.1 specification. But my recommendation is the Computer you either build or buy should have 'both' a USB 2.0 and Firewire interface. The reason being is in case Keyboard Manufacturers start making their Keyboards with USB 2.0 and/or Firewire interfaces you will already be good to go on your computer if and when they do. And USB 2.0 and Firewire Cards/interfaces are very inexpensive these days.

The Korg Pa1xpro has a SPDIF Digial Output (Stock). The Motif ES has an option where if you purchase their AEIB2 add-on Board (or maybe it's the mLAN Board??), it will give you an SPDIF Output. But your looking at extra money for the add-on Board.

Answer to second question:

Here's a top of the line Professional Sound Card that you may find to your liking: http://www.samedaymusic.com/product--AARQ10
NOTE: This Card allows you to input directly to your Computer 'without' the need of a Mixer.

Answer to your third question:

If you're capable of building your own Computer you may want to seriously consider doing just that. It won't necessarily be cheaper but you can pick and choose the Hardware/Software you want to go with it which is very convenient. PS: I am not aware of any Company off hand that sells Audio Computer Workstations but I'm sure they're out there somewhere. You may want to do a Google search: www.google.com and type in the search field - "Audio Computer Workstations" (without the quotes) - and see if you get any hits.

Best regards,
Mike