Let me guess at what you're envisioning. With the new and fast USB ports you've installed in your laptop, you can connect your new hard drive to the laptop and connect the laptop to the wireless adaptor you mentioned in an earlier post. The wireless would let your laptop communicate with your network.
To answer your question about the AX directly, yes, that would do what you want. Like the SB, the AX has a built-in DAC, which you will bypass. The SB also has its own user interface, which you don't need. The SB and the AX, like the USB/SPDIF converter you were thinking of buying, convert data into the SPDIF format--the only function you need.
If you use the AX, you'll go wireless from the laptop to the AX. If I'm not mistaken, the AX only performs its music function when it is serving as a wireless receiver for iTunes. (It has an ethernet port, but that is for use when the AX is serving as a wireless base station. And it has a USB port, but that is for connecting to a printer, with the AX serving as a wireless printer connection.)
Be as concerned about the cable from AX or SB to DAC as you would a cable from any transport to your DAC--but, then again, your wife will be listening from downstairs. If you go with the AX, I don't think "thin" will be the issue with a plastic Toslink. It will be more like extension and resolution. But there are some very reasonable glass Toslinks, like Sonicwave.