RC.....
If they came to you, and they really like you, and if you remind them just how important the Porsche story is around the world, and also the programs other markets licensing opportunities in non german speaking television markets for cable, broadcast and dvd sales..(your english is fine!)
Maybe you should suggest to them that the story can be best told with 10-12 one hour episodes, and that you are willing to do that in exchange for ... standard rate, plus X% of the advertising revenue, X% of foreign licensing sales and DVD license sales domestic and international, plus some up front money for development, and a no cut minimum in case the show is canceled in Germany. I doubt if you would get "library" rights where you obtain ownership of the program after several years of broadcast sales, but you never know!
If you can present the show in English, you might have some pretty good foreign cable and broadcast licensing opportunies.
You really should speak with a entertainment agent in Germany to see how you can milk that DW cow.
Maybe there is enough opportunity for you to saw both ends of the log by getting in someone to take your place at your day job while you exploit the TV programming opportunity.
If you are also known as THE independent Porsche subject matter expert, and not just RC at your business, your customers might admire you for that. Porsche after all is
a German automotive icon. I donno your situation or micro culture.
But if I was you, I would definietly speak with a entertainment agent and maybe in other case, offer to write and produce the program using someone else for the talking head presenter talent.