Haven't found any good answers for this yet...

I've just set up proftp working and ready
Now I want to add more users, I will use useradd command right?
But I want to limit users to only browse my ntfs-drive which is mounted under /mnt/ntfs,
their home dirs and maybe a upload folder.

How can I route these dirs to one specific username??
Should I make a symlink to these folders in the users home dir?

I communicate with the server trough ssh, how can I find the global-ip of the server-machine??
Any good commands for this?

Any other hints on making a fast and stable server with proftpd is cool

you could edit the skel directory so that when ever you create a new user it automatically creates symlinks to the upload and mnt/ntfs folders.
and then you could chroot the ftp user using proftpd.

ifconfig will tell you the ip address of the machine
If you are using a router or dsl modem then you could just setup up dynamic dns and have it update a dns entry then all you would have to do is ping it and it will let you know the ip address.