>I'm setting up Amanda to work as the global ISP solution for customers 
>having to backup hosting/housing/spaces/servers.

Be prepared to deal with lots of firewall/NAT issues.

>I've conviced management that this software can be a great solution 
>avoiding to purchase expensive softwares like Arcserve, Legato, Tivoli ecc.

Absolutely.

>1) to be informed about over quota events in relation to every globally 
>purchased customer quota and even to notify the customer
>2) to effectively deny the backup if the client has exceeded his quota 
>after he has been warned several times
>3) to inform technical staff about imminent tape space shortage

IMHO, these things are properly done via postprocessing of logs. 
Denials of something as critical as backups should only be done manually
by a human, at the behest of said postprocessing.