Use random port for guest SSH to prevent collisions with running machines.
Update URLs to use latest stable debian ISO.
Adapt to debian build host.
Add another reference documentation.
Improve output (to be a little more verbose).
Download latest Guest Additions & fix ISO path.
Remove previously created VM.

Multiple VMs with the same name can lead to unexpected results.
Change defined OS type in VirtualBox.
Disable VirtualBox GUI while installing system.