Decoding music is a easy task, computing wise, especially if you are using direct digital out.
Optical out may be harder to find, many manufacturers are going away from "traditional" laptop models, and focusing more on tablet and notebook designs. The smaller profiles mean less real estate for features. There are converters (hdmi/usb to tos) and off board sound cards that have optical out. If you find a laptop with optical out ( usually it is in the headphone jack, you will need a cable with 3.5mm on one end and TOS on the other (obviously) or a adapter which are cheap and probably the best choice, since you can use your favorite TOS cable) that would be a huge bonus.
1TB is pretty standard for laptops these days (for hard drives) that would hold lots of music (my FLAC files are about 250GB (about 1000 cds). That is more than enough room, but i would recommend using a external HD of some sort( and a back up of all your files as well). I have never had a HD fail on me but I know plenty of people who have. If you are listening occasionally (3-5 times a week 1-2 hours) a usb HD should work just fine and last a long time (since it is not constantly on). The off the shelf NASs are notoriously unreliable, stay away from those.
In the end what you want is a computer that will handle your playback software, and preferably optical out. Don't rule out keeping the old laptop and refresh the OS, uninstall everything but your music stuff, use that just for play back and enjoy your new laptop.
