This should get you VirtualBox, the extension pack, and wget
brew install virtualbox virtualbox-extension-pack wget
When you create the VM, I'd suggest at least a 32 GB "expandable" drive, with 64 GB being safer. Only what you use gets consumed on your Mac's hard drive.
Here's a link to the Debian CD images. You don't need a graphical environment, and I'd suggest adding a host-only interface and enabling sshd in thee setup. You'll need to add sudo
and add yourself to the sudo
group before you can SSH in to your VM.