Applying a consumer/developer image afterwards doesn't work, I don't have the exact message but it reboots continually. :-(

Having the exact message might help us diagnose the problem. Can you get it for us?

Also, try Bitt's suggestion: Use GetRight or some other similar utility to re-grab the files. They might have been corrupted on download.

But none of that happens. It boots to the point I quoted earlier (discarding initdata sections, the "Freeing unused kernel memory: 4k init " message) and then simply hangs. Hitting enter doesn't help, nor does q.

This means the builder image didn't work at all. If it worked, you would be seeing it format the disk and run a stress test. It didn't get that far.