Cheers!
Having done a bit more homework, I've decided that the best option is just to get the fastest possible PCIe soundcard I can, and run a mixer off it for my analogue I/Os. This means I no longer have to worry about anything other than the performance of the soundcard itself, mainly with regards latency. As far as I can tell, MOTU and RME are the way to go here... any recommendations? Again: no need for a breakout box now, I don't think.