Joni
Look, I touched this Plone site - from a system/setup point of view - the last time about six years back due to other priorities in our business which is mostly not Plone nor LINUX at all. This was a mistake, which I see now clearly.
But due to the stability of Plone and LINUX and the content which is in the CMS which is heavily used daily, I'd rather not change the plattform.
That means for me (lessons learnt) to more often touch it and bring it to the newest version.
BUT: Even then, I won't ever become a LINUX/Python/Zope/Plone-crack and therefore look for the way of doing things in a way, that I can do those updates with the least frictions, trying to avoid conflicts as good as possible from scratch. That's why ask this way. This goal cannot be reached completely as no one of us can look into the future, but I can at least try to do it that way as good as possible.
So: Yes, you are right, absolutely, but I still look for ways to avoid obvious problems from the beginning 
I thank you and Thomas very, very much for your help!
It seems, that I could update my Plone from 4.2.5 to 4.2.6, where I had the troubles with buildout. I unpinned both and now buildout ran through. I then could update my Plone-site 
Unfortunately I cannot do it on the old Plone with 4.2.5 (but Python 2.6) that way.
(No activate-script available, tried to install the Python-package virtualenv but didn't work as I expected...)
Therefore I will now to take both sites to 4.2.5, try to configure the site on the new server with the same configuration as the old (LDAP for domain authentication, Theming, ...) and then transfer the content from the old server to the new.
After that I know now, that I can update the site further on - and will see, how far
So for the moment, I thank both of you very much! If you still have an input with this, I will still follow this topic. If I am further down the road, could import/export my content, but get stuck again, I will come back to this topic again...
Thank you again and kind regards
David