From what I recall, there was no standardised type, with various Mercedes Simplex models and others being used. Too bad you only bring it up now, since I own a Tankograd Publishing mook on the subject (actually, it´s about "Fahrzeugraritäten des Heeres und der Wehrmacht" from c.1900-1945, but the WW1 sections are the best part), which, unfortunately, is now lying in storage at my parents´ place in Germany.
From what I gathered from the pictures in the mook, you could either have them a greyish green or leave them as is, but as for the Daimler: style-wise, I´d say it´s perfectly ok to use such a car, but I would strongly suggest repainting it Feldgrau or greyish green/greenish grey as to make it look more "military". Maybe add a maps folder or some other nice details.