Can I use a Time Capsule as an all-purpose storage disk?
Yes. Time Capsules can be used for Time Machine backups as well as for general storage, though Apple recommends against it:


  Time Machine works best if you use your backup disk only for Time Machine backups. If you keep files on your backup disk, Time Machine won’t back up those files, and the space available for Time Machine backups is reduced.


I personally don't believe that they will “work better” if you only use them for backups—I haven't had any problems with my setup, and it's been this way for a few years—but that's what the official knowledge base says.

It's easy to get at the general storage. I have disk accounts set up, so the way you do it might be somewhat different. My Time Capsule appears in Finder's left sidebar. When I click on it, two items appear: one with my username and one with the Time Capsule's disk's name. I double-click on the first one, and (after waiting a few seconds for the disk to spin up again,) I get a list of the files I've stored there. Finder also puts an icon on the desktop, which I can use to quickly get back to it. When I'm done, I either drag that icon to the trash, or click the eject icon next to the Time Capsule's name in any Finder window.



Time Capsules do seem to support PPPoE for Internet connections: in AirPort Utility's Internet tab, there is an option for PPPoE in the Connect Using drop-down box. I can't be 100% sure of this, though: my ISP doesn't use it.



(Everything here was copied from my three comments.)