You can install from the dvd / cd version (once the cd version is ready) and download it or a copy can be send by post to you.
Updating using emerge / binmerge / entropy ( last one some time in the future option ) can be done offline provided you have somehow downloaded the files and placed them inside the portage folder /usr/portage/....
Most of the time if you have the dvd the files are fairly up to date and only needs something small like mozilla-firefox update = 1 or max 10 small programs at most including a few others you want to update.
For most part there is lots of files on the dvd to do the install and get it working.
The only real reason to update is to get it so much customized to your pc to get max performance out of it and so basically rebuild the whole thing or update if there is a problematic program / security update.
95% of the time customizing it is not really needed = fast enough or you would not really see a speed increase.
Hope this helps.