* A good solution would be to make a list of download links.
* Add some mirrows, but save it in a way that programms can easily read it.
* Add checksums to check your local files.
* Make a JavaScript, C-Script or more than one script that downloads all failes with the informations given in this link list, but skip maps which are in your map folder.
* Can a torrent programm be configured to skip existing files? Then make a tutorial for them and a fitting torrent-file.
Then I could rethink about downloading a map pack where I have already more than half of the files.