First of all, thanks for your offer to help.
Currently the full VideoLAN archive is around 8GB big (it slowly grows, every time we release something).
We ask our mirrors to synchronize at least daily using some command line:
rsync -avz --delete
anonymous@rsync.videolan.org::videolan-ftp /path/to/directory
Estimating your bandwidth usage is kind of complex. We currently use a random redirection script for most downloads so users are more likely to use mirrors near them then far from them. The redirection script also takes into account the advertised mirror bandwidth. Since you're offering a mirror in Germany, you should be pretty safe from that point of view since we already have lots of European mirrors so the load is nicely spread-out.
About the protocol, we accept both FTP and HTTP mirrors although we prefer HTTP since that's the only protocol which we use in the automatic update feature in VLC.
I hope that this answers your questions. You can always setup a mirror and give it a try for a few days to see what kind of bandwidth usage you'll get.
Thanks once more.