Is there any way to configure the Dropbox app so that all Dropbox files, now and future, are available offline on the SD card?
Yes, you can
Specifically, making a file available offline:
Adds the file to your Offline view for easy access (like a bookmark)
Saves the file to your Android device for offline viewing within
Attempts to keep the file as up-to-date as possible and gives you the option to update the file manually
Ensures that any changes you make to the file while offline will be
synced to your Dropbox account once you have a connection again