Thank you all again for your help!
A friend of us living in the USA asked around, and meant that it would be no problem for us as under 21 Germans to buy a car there with buying, insurance and registration. Is that right or may there be problems?
Do you have any recommendations of a car, it should be cheap in cost and gas, have enough space for us to sleep inside and reliable that we wouldn't have to repair it.
What car would you recommend, what do you think will be the costs for a solid car, insurance and registration? When we had a car it could be a one or two months roadtrip, with a rental car we could only afford two weeks.

Thank you for your help!

Jannis