FWIW, I use a variation of option 1 and love my Squeezebox Classic and Boom. No stinking i-Tunes in the mix potentially screwing things up. Since the SB Classic and Boom UI only allows me to view a single line of information at a time (band, or album, or track, depending on which level of the hierarchy I'm in), they work best for finding music I know I want to hear...not so much for browsing a collection. If I really want to browse (which is rare), I can do so on any computer on my network with a browser. Additionally, I can sync the Classic, Boom, and SqueezePlay on my office computer to all play the same thing at the same time, which is handy when I'm moving around the house a lot while listening.
If I were to do a similar setup with the current offerings, I imagine it would be SB Touch for the main system (digital out) and SB Radio for other rooms.
As for backup, I'm inclined to keep it simple: add enough storage to your old PIII to maintain at least 2 copies of everything and schedule rsync on a nightly basis. This is what I do. (I think RAID is overkill, as well as risky and problematic at consumer price points.) Unfortunately, I can't provide any feedback on how to address universal access.