(as root or sudo)
will shutdown immediatly
Will reboot immediatly
I expect reboot (which most distros have) is just a script that executes the above command. Check the man pages if you want to schedule it for different times etc.
As for remote admin commands well... SSH is a Secure SHell, meaning you can do anything in that shell that you can locally.
you could use webmin to setup things via a browser etc. it all depends on what you want to do