I never tried to tour the Mercedes factory, after all, they have so many of them throughout Germany. But I did drive across the city and tour the Mercedes Museum and I'm glad that I did. If you want to see the better museum and have only limited time, or perhaps a limited view to your enthusiasm from your spouse or other travel companions, I'd suggest the Mercedes Museum as it is much better.
When you have arrived at the front door by shuttle (see in directions below), you will walk in and to the right, you will find a staffed kiosk where friendly and helpful attendants will provide you with a recorded tour in the language of your choice. I cannot remember if it was free to use this item, but I think it was. Forgive me if I'm wrong about that. Anyway, this thing is controlled by you, as to the pace. At a certain point, I chose to turn it off until I found something I wanted to know more about. Whatever your particular Mercedes interest, you should find some representation of it here. There were way too many of the first years cars for me. But all of the old Formula 1 cars and road cars and everything else was here. The museum is stylish and easy to navigate. There is a nice boutique and cafe (and nope, the food and drink is not expensive). This is a museum and there's not much for small kids unless they like cars. So if possible you should avoid taking them, although the museum is kid friendly. I think they do have a rule about keeping them with you at all times.
The museum is closed on Mondays and public holidays, but is open Tuesdays through Sunday from 9-5. The parking is free, the tour is free. The layout is about 3 stories high, but the layout uses ramps that you walk up and more cars are laid out along the way. Individual tours can be arranged, but you'll need to call first.

