You just need a toslink>3,5mm optical cable and you're good to go.
Don't know if anyone made a side-by-side comparison, but I'd imagine that at least modded UA-5 units would come out favorable, compared to the internal pre+AD.
Though the real benefit of the UA-5 is phantom power, and the ability to run matrix. If I just had a D50 and plain 3,5mm battery powered stereo mic, I probably wouldn't bother with an outboard box.