[Klug]

Hi Thomas, welcome to the forum.

I'm sure you'll find Germany based consulting company to help you but I really think for 6 users the "having your own server" is overkill (and overcost).

If you want roadwarrior usage, that means (currently) using Outlook 2003 as client (to have offline mails and calendar).
So it means going with Zimbra Network Edition Pro, and it means 25x35 USD per year just for Zimbra (25x25 USD if you don't want Outlook 2003, thus roadwarrior for mail/calendar).
Add the server, the paying OS (to have a supported OS) and service...
And add Zimbra Mobile too (500 USD per year).

Most HSP will bill you between 7 and 12 € per month per user for the same service.

About your question about antivirus and antispam and SmartPhone : what you want is the way Zimbra (and Zimbra Mobile) actually works. Messages pass through the antispam/antivirus and only "nice" messages will arrive to your smartphone.

The Bat! (that works beautifully) is able to deal with IMAP so you'll be able to import all your messages to Zimbra easily.