Reselling hosting to reseller's only should require less support than just selling hosting plans to individuals; but wouldn't just reselling "managed servers" require the least amount of support? Is there a company that lets you resell them AND they do the managing and tech support for them. If so, who does that?

What about un-managed Unix (Sun Solaris/FreeBSD) or Windows 2000 servers? How difficult are they to manage? I'm mostly familiar with FreeBSD with Apache.

Finally, do most servers come with a free pre-installed OS and Control Panel?

Actually,
In my experience I generally need to provide much more support to our reseller clients than regular single plan personal clients. This makes sense as they represent many more domains & clients, and therefor naturally have more issues to be solved.
For a managed server, you can expect a great deal of support at the start and less as time goes on (unless a major problem appears.)
Unmanaged would obviously require the least amount of support time.