Thank you for your reply!
It's really good to see that I'm not the only one intending to use Kolab in this scenario
As far as fixed IP is concerned, I thought a dynamic DNS server would solve this problem, but again, not 100% sure about it.
Did you also try to import old mails into Kolab? That would be a special requirement for me...
Also, how powerful is the machine you're running Kolab on? I realized my BananaPro could be a little too weak, but apart from this I have no idea how resource hungry Kolab can be, especially if I will use the file-cloud server as well as contact- and calender-server.
thanks again for your help and hints