2 cores helped indeed, thanks! - I struggled with a too small disk. If the disk is too small the installation process hangs forever without giving any error message. My assumption is that the dynamic sizing performed by Virtualbox doesn't work in this early stage when there are no Guest Additions. I first had to adapt the disk size accordingly until I had the first VM running.
Anyways, all I need to do here is to check whether the installation scripts work. And another pain somewhere in the Galaxy are exactly these Guest Additions, I always spend 10 to 15 minutes until I get them going and can reboot, having a reasonable display then. I should write down a recipe on a 3M Post-It-On-Display™.