To enter the Autobahn your vehicle must be able to drive at least 60 km/h. As far as I am aware there is no minimum speed but you also mustn't be a hindrance to other people. I believe if you drive less than 90 km/h without a reason and you're involved in a crash it becomes partly your fault.
regarding death toll: from
http://www.sgipt.org/verkehr/vus/vus_2007.htmIn 2007 there were 4,949 traffic related deaths. On Autobahns died 602 people (-6.7% in comparison to 2006), on country roads 3,012 (-1.6%) and within city limits 1,335 (-3.5%).
And even on german Autobahns there are very rarely people driving Ferraris with 300km/h. And those are mostly foreigners (from Switzerland, UK, US) who want to drive fast once in their life.
The average driving speed is somewhere between 120 (75mph) and 180km/h (112mph), with peaks to 220km/h (137mph).
My personal high scores are 214km/h (133mph) with my own car and 230km/h (143mph) with a rented Mercedes.