Get the Sequential Circuits' Prophet-5, I saw it in Dieter's home studio on some rare  (and small) photo (you may find it on the net on some fan's pages), also there was some other synthesizer, but hard to say which one as it was photographed from the side, possibly an Oberheim. These are likely where used on first Modern Talking albums. You can't really get these sounds on today's computer based emulators, maybe something close but the character will differ from originals (which in turn generate sounds using a complex “natural” electronic sircuits made from minerals, just like acoustic instruments use natural wood and metals constructions to make sounds).

Regards,

Konnoo Records