Firstly, +t for wanting to get the best possible quality out of your recordings. That's definately the right attitude!
Personally I don't think you'll gain anything... you can't magic an extra 8 bits of quality out of thin air... as you rightly point out MD is only 16-bit.
One option to maybe think about: I bought a sony MDS JE-470 specifically to help a friend transfer a MD master. I bought it off ebay, and it was really quite inexpensive. Despite not having optical out, this unit had space on the circuit board for an optical out connector (in fact it turned out the same circuit board was used for the higher-end MDS JE-770). Once I had the components it was an evenings work to drill a hole in the rear panel, solder the bits on AND do the transfer. I've been meaning to write more about this for a while... if there is interest let me know.
Having said all that, if analogue is your only option right now, it makes perfect sense to use the best quality gear in the chain you can. The ua5 sounds like a good choice to me. Just run everything at 16-bit and be happy.