Helo Fenna and welcome to Playmofriends from me in Stroud, England.
I am sure we will be able to help you with your thesis!
Scans of all German DS catalogues are
here and a catalogue of the contents of most of them
here.
I don't have any links for scans of main catalogues. I do have actual German catalogues from 1995 on and UK catalogues from 1981-2 on. But I think scanning those would be very time-consuming, so hopefully you will find someone else has already done it.
Amclicks (Spanish) and Klickywelt (German) are two playmobil forums with far more active members than Playmofriends so it might be worth posting requests there.
Please feel free to make use of anything on my website which you find useful.