Hi,
RME has a few options for doing this at different budget levels.
you will need a PC with enough fully functional PCI or PCI-e slots or a MAC Pro with a PCI-e expansion chassis.
For software most Major DAWs will work. Cubase, Nuendo, Samplitude, Logic, Sonar, Reaper will all handle pretty much as many tracks and I/O as your computer can deal with.
You will need a Raid array to do this. A single drive will not be fast enough. A single SSD drive would be fast enough but storage capacity will be an issue.

1.
3x HDSP MADI PCI/PCIe cards
1 or more ADI-648 MADI to ADAT format converter
8 or more 8 channel ADAT Mic Pre-AD/DAconverters. The RME Octmic II  and/or the ADI-8 DS are a great choice for this but any brand 8 channel AD/DA will work for this.

2.
3x HDSP MADI PCI/PCIe cards
8 or more RME Micstasy-M Mic Pre and/or ADI-8 QS-M 8 channel AD/DA. This will be the highest quality and more flexible but will of course be a higher cost.

Either of these combinations will be cheaper than the average digital mixer or even most I/O and stage boxes available for those digital mixers.

Depending on how soon you will be doing this there will also be the new MADI FX card will will simplify the setup as well as lower the overall costs by putting all the I/O on one card instead of 3.

Thanks Chris