This is the exact way I have my cisco gear currently setup.
To answer your questions
No it does not have to be a domain controller, I can't really think of anything compelling for/against. I have mine on the DC's local to the equipment because a) they are already doing authentication and for network gear auth you won't be adding that much load and b) didn't really have anywhere else to put it at the time.
Nope won't cause a reboot or start/stop of services
The first guide looks pretty much spot on for the windows side. See below for example code from my configs to get the cisco side running.
One thing you want to do is make sure you have local accounts that you can fail back to when your radius server is unavailable. and make sure you have an active console session that is set to not timeout when setting this up. You can easily lock your self out of the router while doing the setup.
This is for authenticating console access not VPN access, i'm going to leave it here in case someone finds it useful.
aaa authentication login default group radius local
radius-server host <server_ip/name> auth-port 1812 acct-port 1813 key <encrypted_shared_key>