You have a perfectly useful rig right now, you just need an adapter to go from the transformers to the iRiver's line-in.
Then benefits of the UA-5 are that it does phantom power and analog to digital conversion in one box, along with level adjustment, and it sounds good too. So when you get it, your rig will look something like:
SMK's > XLR > UA-5 > Optical Cable > iRiver
You won't need the phantom supply anymore, since the UA-5 provides that for you, and it will also likely eliminate the need for the transformers, but you should keep them around in the event of a really hot situation, like a hyper soundboard feed. The UA-5 also gives you more inputs, so you could run multiple sets of microphones (for which you would want to keep the phantom supply) or a soundboard+audience matrix.
But by all means, get yourself a few dollar adapter and go tape some stuff now. Get a feel for the iRiver and see what you like. +T.