For CS:S(or any source game) it would be a good idea to have a different IP for each server, so if you define a hostname (css.aclan.com) clients do
not need to specify the port, they can simply type connect css.aclan.com in console.
I would recommend managed for just about everyone, it makes the job of running servers alot easier, and NFO has some great managed utilities, including but not limited to:
Automatic game server updates
Automatic server configuration
Auto-installers
Advanced firewall gui
Managed also allows as many IPs as you make servers. If this is your first server, try buying something a little smaller to start, like a 3 or 4 core, theres no sense in having 8 cores with 1 or 2 popular servers
Good luck