Rather than going all out emulated hardware with VMWare, VirtualBox, *EMU, I think you should consider the OS-level virtualizations available. Linux-VServer, OpenVZ, or LXC will all provide secure isolation to each guest, near native performance with almost no overhead, and allows for much easier management in my opinion.
I have personally been using Linux-VServer as it has a longer track record.
In regards to your question of VBox though, the guests would be reachable via ssh (providing the sshd is started and listening). Or whatever other protocol you wanted to listen on such as VNC. I can't speak on VBox's RDP support though. However if the ssh daemon for some reason stopped in the guest, I have no idea how one might administer it (actually this is most likely what the RDP support is for).
In contrast this would be easily solved with Linux-VServer because each guest is essentially a super chroot jail. You can literally 'enter' the guest at any time as root from the shell.