"…you consider dakhla to best the best place to find a spare seat." YES
"I think it should be possible to get there by bus isn't it?" YES
"Another question: do you think I should get the visa for MAUR in Germany before leaving or in Rabat (I heard it's cheaper and faster there)?" I would suggest in Germany, in your own language, and in you own time, but check around, Rabat may well be easier – do they still ask for a return air flight as a condition of entry? Worth doing some more research on this. My info is not current and there have been many changes in the last year…
"is there also a visa-office in dakhla?" It is not a visa-issue office, it is a permit-to-travel office authorised by the gendarme where your exit-visa is stamped.
"how much … for a seat in a car / bus from Dakhla to NKT / Dakar, just to have an idea..." I don’t know, but to offer to share the cost of a guide for a good start. About 3 years ago a guide cost around $200, I think.
If you drive the beach route between Nouadhibou and Nouackchott, this will take around 3 days. There is no piste, lots of sand and some local quicksand. I believe a guide is essential. It’s also a good trip through the Parc D’Arguin, with coyote, pelican, fish eagles, some small dunes, dolphins in the sea, washed up whales, lots to see!
Note you need a permit to travel through the Parc d’Arguin, about $5 from Nouadhibou.
I haven’t travelled this way for a while so other travellers may add further info...