I think everybody is getting ahead of themselves here... let me try to clear up a couple of things:
- The USB device your dad is using is, in fact, an external audio device, also known as a "USB Sound Card". Even if it were designed for recording as its primary function, if it can output audio, it *should* be just fine for this. As long as the drivers are up to date and Behringer still supports the device... which can be a very big "if"...
- Most (damned near all) mixer boards don't have a direct digital interface. I actually do have one, but it's a Fostex Hard Disk recorder that happens to have an SP/DIF optical input. For the most part, you'll be using an analog 1/4", XLR, or RCA input from your computer. I know they also do make mixer boards with direct USB interfaces, so it acts as a sound card, but they are quite expensive, not well supported, and in my opinion unnecessary... plus they usually have "scratch tables" which is an entirely different DJ style than what I support...
Bottom line is this: I personally use a Sound Blaster USB MP3+ device. It does have optical, but it also has RCA jacks (2-in, 2-out) and even though I bought this thing in 2002, it works great.
When I look at sound cards or external audio devices, I look at a few things. For one, you definitely want RCA or (better yet) 1/4" jacks. You want gold plating - it really does make a difference. And you want a device that is *supported* by the company that put it out. Any company that says "well we made that 10 years ago, so go buy a newer one" is not the kind of company I would do business with. They should stand behind their older "legacy" products, and make sure they will still work in the next version of Windows, on the newer PC platforms, etc. Otherwise you're just wasting your money...
I don't know much about Behrigner's USB audio devices (though I do have an 8 channel mixer and a couple of compressor/limiters from them), so I can't comment on the quality of their drivers. Just know that if they don't provide support for older products, they will most likely phaze out any new products you buy from them (this goes for any company). Just something to think about.