I am in the Auto industry. Webuycars do make it clear in their terms that they "do not warrant that a vehicle is in a good working order", or that it will be"usable and durable for a reasonable period." If you main point of contention is that the car does not run you will get nowhere.
Knowingly falsifying a Dekra report is fraud... You have a chance with this case with the Ombudsman if you just focus on the misleading Dekra report. But chances are they just don't do proper inspections. They are known for this!
But Dekra is an independant 3rd party comparty contracted by Webuycars to ̶h̶e̶l̶p̶ ̶t̶h̶e̶m̶ ̶s̶c̶a̶m̶ ̶p̶e̶o̶p̶l̶e̶ do damage reports. Legally, this makes this complicated as this means Webuycars are not necessarily responsible, and your problem is actually with Dekra.
As with past cases similar to this, you need to contact Dekra. Preferably with a lawyers letter.