Q1 - Do you want the files on both PC and NAS?
If so some software backup solutions can be had with one button backup or start-up/shut-down automatic backup options. In this instance you would rip to the PC as normal and ensure the backup program includes your PC music store in it's default backup path. The hit the button or log-off to trigger the data transfer.
Western Digitals MyBook drives come with said software (I forget the name*) as a trial and it's very effective.
Q2 - Just based on the drives and from an IT perspective I currently recommend Western Digital or Samsung based solutions. For portable/network devices that means just Western Digital's own solutions as I don't know which externals use Samsungs current drives.
Q3 - Do you need DNLA compatibility?
I have a dedicated PC for media so I can't comment directly on Squeezebox compatibility. I'd suggest a quick visit to the forums and hunt for threads on your chosen drive incase that throws up some anomalies.
BTW I assume you'll still have to run your PC for the software. If so why the sudden need for an external drive for anything other than backup? Please correct me if I'm wrong because if I am I'll be asking for a portable from Christmas! :)
* Just noticed it's called Memeo
Panasonic TH50PZ80
Arcam Alpha 7 CD | Sony BDP-S760 | Xbox360 | Wii | Virgin+HD | Squeezebox Duet
Arcam Alpha 9 Int | Yamaha RX-V1800
B&W 602 | B&W CC6 S2 | Tannoy 603 (Front/Centre/Rear)
Stands Unique Ultimate High & AV10 High