Hi,
Been working with asterisk for 5 years or so now,
Have written entire management systems for it so have that side covered.
Plenty of backup solutions out there, something like backup pc might be applicable, depends if you have a backup server, or want to push backups to a storage medium.
Maintenance is easy enough, will run you though.
Web servers are easy, Apache is good, but if your going to do really high load something like nginx might be better.
You need to be very very careful with security, server should be behind a firewall, with restricted outbound where possible, also software like fail2ban is vital! (will automatically ban people trying to break in)
Fraud is BIG in voip at the moment so be prepared!
Performance can be monitored in a variety of ways, cacti is a nice monitoring engine, draws graphs and such.
What distro are you using for the server?
Id recommend CentOS but others are good.
Nick