The budget version would probably be a firewire soundcard with about 8 analog inputs and a SPDIF input, plenty of these around to be bought. If you go from the mixer we are talking line level so preamp not needed (as you use what is in the mixer). Add to that an external AD converter translating to SPDIF, lowest price around is Behringer ADDA8000.
From there you can go upmarket in quality and probably a bit in stability. Personally I would exchange the lowpriced sound card for what I run now, a RME Fireface 800, as it has proved to be rock stable.
Gunnar