For DRC Visa try Libreville, my travelling buddy at that time got a one month visa there for 40.000 while I payed 100.000 in Yaounde for a 2 month with loads of hassle and I had to swear I never to to Kinshasa with it, uppsss ....
The big problem with DRC is, that its possible they reject you at the Brazzaville - Kinshasa border cos you havent been resident in Gabon, happend so a couple of times to travellers. Because of that we entered DRC 3 weeks ago thorugh a tiny border post ( bamboo hut in the middle of nowhere with a guy that had to walk to the next village to get entry stamp ) near Mindoulli. The road going there is terrible, Ekok - Mamfe is a complete JOKE compared with that one, I doubt even a Landcruiser 4x4 can make it without a big risk of getting seriously stuck. The piste is going through Miyamba - Sundi Luttetete - Mangembo to Luozi where you take a barge. From there to Kimpese is again pretty bad and youre stuck if it rains. This road is hardcore and has close to no traffic even maybe just 3 trucks a week, I know it because my chassis broke and I had to wait a week in the catholic mission in Miyamba to find transport to put my vespa on the roof, in the end it was the priests jeep cos non of the 3 trucks that week went all the way. Forget this road in a car with 2 wheel drive and low ground clearance. I did Ekok - Mamfe and in direct comparison its a excellent road, its extremely hilly there and sometimes even the Toyota didnt get further and we had to get stones and shovels to work on the piste ...
I would consider going the Franceville road from Gabon to Republic Congo. The border at Ndende has really really bad sections and if it rained the last week you will definately get badly stuck and the only way to get out again could be hiring villagers or another car ( little traffic ) to pull you out. The tarmac stops at a big before Ndende and just starts again when you reach the junction at Dolisie, the piste is very rocky and pray it doesnt rain.
From Dolisie to Pointe Noire is perfect new tarmac road by the chinese, the best road I have so far seen in central africa. From Dolisie to Brazza is under construction and complete piste with some rocky nightmare sections around Nkaiyes. The locals do it with all kind of cars but oh boy, I never see a chassis suffering and making noises like that, from what I heared Franceville is the overall much better option.
Angola visa on the road is definately impossible in Cameroon, Gabon and the Republic Congo. My friend tried very hard and these guys are complete dickheads with NO interest of you getting into their country, me personally i went to Dolisie and Pointe Noire consulat for translating french for him but not a chance even with invitation letter. Can t comment on situation in DRC but I dont think its better, leaves only the option of sending the passport to Berlin by DHL, best done from Gabon or Cameroon. Dont get stuck in DRC doing that, as the guys already said you can ship the car from Pointe Noire if everthing goes wrong ...
greetings from Kinshasa from a fellow german traveller,
( Hamburg - Kinshasa on a 1975 vespa scooter )