What more hardware do I need for REW?

I need to get REW up and running.  I need advice and recommendation for an appropriate Analog to Digital converter.

I have a good Alienware laptop.
I have a good battery-powered portable outboard DAC (iFi micro iDSD USB to stereo RCA).
I have a Radio Shack sound level meter.

I have a decent but cheap mic preamp with phantom power.
I have a Behringer measurement mic and a mic stand.
I have a Behringer "digital audio mainframe" that has a sine & pink noise signal generator and RTA.

So I need some kind of A to D converter unless I want to just use my sound card. At some time or other I'm going to want to record to a PC anyway.  I just have no idea what brand/model, how many channels in & out, etc. I should be looking at. I have big mixers and little mixers and preamps. What I don't have is any A to D at all, except the sound card.

Should I just dive in and start using a sound card?

Should I get something intended for multichannel recording, figuring I'll use it later for more than REW?  One or two channels, or is 4 or 8 the sweet spot?  My Allen & Heath is LCRplus, 4 group, 8 aux, 8×4 matrix.  So maybe there's some synergy in being able to record live to 8 channels from its 8 VCA groups.