I have had surprises with Hertz in Germany both ways. I usually book MB C-Class/BMW 3/Audi A4 and worst I ended up with was a Kia Sportage, best I ended up with recently was an Infinity FX35. I found that airport locations are usually better than non-airport locations, since a lot of non-airport location are franchises. I generally prefer Sixt in Germany - they are the #1 and tend to have more cars and better variety.

As it relates to navigation systems, I think it's not necessarily reasonable to expect station employees to know all the different navigation systems in different cars, since there is no standard "Neverlost" as in the US. I remember having to unpack and install/set up the navigation system in a brand new Volvo I got from Hertz in FRA. I also usually bring my own navigation system since I don't want to pay an inflated daily charge.

Cheers,