If you bought the game on Steam, started the download and then paused it, copied the disk install folder contents to the Steam install folder (the download version got rid of the 'run' folder and moved all the subfolders up one level) and then verified local files for the game, I think that would limit the download size.
The GOG download is a single 1.83GB file, though using the GOG downloader you can probably pause the download and resume it later (not sure how it would handle waiting a month to finish). You might be able to download it onto a flash drive from work or a friend's house.
Some people have reported compatibility issues with disk version saves in the download version of the game (either crash on load, or the saves would load but saving would be disabled after that).
When Larian went to update the game for the anthology there was a problem with the backups, so they ended up loosing some of the bug fixes and changes that were in the disk patches (blog post
Hatching the Anthology). These are mostly minor issues (for example, you can not return Finnigan's lockpicks to complete a quest and then buy them back like you can in a game started in the latest disk version, and the lockpicks will disappear if dropped on the ground) or infrequent bugs anyway, or ones with other workarounds or fixes.
List of known bug(s) in the Steam Version - the music bug in the download version was fixed, some of the reported issues in the topic are also applicable to the disk version or are rare, system dependent or not actually bugs.