I just left Cameroon a couple weeks ago and talked to several Germans who were traveling around without transport.
The public transit is very good with Large busses in the North and South. There is a train that runs across the middle section.
Cameroon is pretty tourist oriented so you should have no problem
As far as budget well the sky is the limit.
Expect to pay about $40 - $50 a day to rent a landcruiser with driver to see the Parks in the North. Obviously buses are much cheaper.
Hotels were a little spendy, expect $20-$30 a day for one with a hot shower.
French would be very helpful but it is a littel easier than the other Francophone countries since Cameroon is actually a bilingual country.