Post Build Hardware support

My son needs a MAC pro for his music... lets just call it stuff. He's quite happy with a Hackintosh and I'm quite happy to build him one.

I do have some minor concerns however. I will be going with all the recommended components so I don't expect to run into too much difficulty building it and getting it working. My concerns lie with what happens once he starts using the unit and connects his external devices to it.

His current sound card is what I can only describe as a completely external unit that is connected to his PC via a USB or Firewire (I'm not sure which) cable.
Yes, the sound card on the motherboard works just fine, but he only uses this for every day stuff... surfing gaming etc. When he is doing his serious music recording and manipulating, he uses the external sound card - a 1U high rack mounted piece of kit with endless knobs, sockets, buttons, and an array of LED's the likes of who's function I can only speculate on; connected as I mentioned to his pc via a USB/firewire cable.

I am not certain of the manufacturer, though I can certainly find out, but am rather more worried that this sound card along with his Music keyboards, and other perpheral devices will not work when he connects them (all of them have USB/firewire interfaces) to his new Hackintosh monster (which he is becoming decidedly excited about - it being off the beaten track from what his peers use, plus he loves the Lian Li case I have chosen for it which makes the Mac pro case look positively pedestrian).

At any rate, I suppse I am asking if there will be complications with him using external devices such as this. If they will simply install on the Hackingtosh with the drivers provided on the install CD/DVD disks they are supplied with (I notice they all have Mac drivers and software included as a matter of course) then I am quite happy to go the 'whole hog' and build this cool bit of kit. However, if I/he am likely to encounter problems using anything outside the reccomended standard of suggested components then I may just have to fork out the Mac pro bucks.

Your advice on the matter is greatly appreciated.