Emil Schult was the guy's name who did alot of the actual Custom Electronics for Kraftwerk. If you look closely on your
Kraftwerk Albums/tapes/CD's , you may see a credit in his name.

Kraftwerk doesn't talk much about their gear. A lot was custom, some were stock synths with mods. When synths were analog, you could actually mod them fairly easily.

Their classic period (pre-Computer World) gear included a Minimoog, Micromoog, ARP Odyssey, EMS Synthi, Prophet 5 (you can see it in the video for The Model) and of course, the Senheiser vocoder. Some of these you can actually see on the album cover art.

They didn't really have optical discs for audio back in the 70's, but they still use old Sony videodiscs on stage for the visuals and have been since the '81 tour, I believe.

As for the strings, in the 70's that type of sound was usually done on a mellotron, which had dozens of little tape loops, one for each key. I think Kraftwerk used a similar tape-based device called an Orchestron rather than a mellotron.