[Windows Phone 8 Q&A, Help & Troubleshooting] GoodDayToDie:
That's a great overall description! A few more comments:

1) The phone should be able to *open* any Office document just fine, but you won't have anything close to the full Office suite's power to make changes; it's mostly basic edits only. For example, you can add or delete slides to a Powerpoint deck, and edit their text, but I don't think you can create or edit custom themes.

2) Documents, music, and pictures are no problem. The phone connects via Media Transfer Protocol (MTP), same as most modern Android phones do; all major OSes can access it, but it is *not* the same as USB Mass Storage. You can't just use it as a USB drive. Additionally, this kind of access only works for the built-in "Libraries" (in the Windows "My Documents", "My Music", etc. sense) on the phone; if you, for example, use a third-party app to handle a file type, that won't show up. One example is ebooks; you can open ebook files on the phone if you download them from the web or open them from attachments, but you can't just copy a bunch of .MOBI files into (or out of) the Kindle folder, for example. Note that this assumes no special hacks; we've been able to get full filesystem access on Samsung phones (such as the ATIV S, which I have).

3) Generally, opening any kind of attachment is possible. If the phone doesn't have an app to do it yet, it will offer to search the Store for compatible apps. If it has multiple compatible apps, it will ask which one to use. However, where attachments get saved is up to the app. The built-in Office programs and image viewer can save to the system libraries. Not so sure about videos or music, although they will open using the built-in apps (for recognized formats, at least).

4) Yes, using your own music works fine. Copy the clips to the Ringtones "folder" over USB, or use one of the many apps (they can do things like trim the file for you, too). Some notification types require GDR3, which your phone may or may no come with but which you can upgrade to easily.

5) IMAP works great. Switching folders is a *bit* more annoying than I'd like - three taps - but it works, and you can control which ones automatically sync to the phone. I use a private IMAP server without any trouble.

6) You really *should* have different user profiles on the PC (for unrelated reasons), but the phone OS doesn't require it. I don't know for sure how well the "Windows Phone App" handles the situation, but I do all the stuff manually anyhow (using Windows Explorer and other tools) and that works fine with multiple phones.

7) No way to know for sure. It's pretty well guaranteed that a phone released so recently as the 925 will get the update, though, and these days Microsoft allows developers and enthusiasts to get updates without waiting for them to finish carrier testing and customization (you'll get the customizations once they're released too). T-Mobile US is pretty good about releasing updates anyhow, though, and the phone's specs are easily good enough.

If it helps, the Samsung ATIV S (SGH-T899M, not the other models) works great on T-Mobile frequencies. The only problem I've had is with the WiFi tethering (USB tethering is unofficial but works fine and is built in if you can find it; instructions are on the forum) and everything else works including LTE. Can't get the loan from TMo for it, but you can find a SIM-unlocked one online for cheaper than the 925 anyhow.