Start in Germany, and go further south
Especially if you don't know too much about cars, It's gonna be way easier, cheaper, and probably faster too to buy a car in Germany and drive it to Addis Abbeba or have it shipped to somewhere on the east coast.
As for your route, unless you're intimate with the region, I'd recommend to go further south and approach congo trough Angola.
If I did it again, I'd drop the cooler. I didn't find it very useful in Africa. Most people don't have coolers, hence, there's not a lot that needs cooling to buy along the road.
A rooftent is nice.
In terms of preparation, if the car is sound, the only thing I'd consider a must is good tyres and a toolbox.