Windows Server Monitoring and other OS in extras

Hello All,

i'm searching for a while now a software that can "monitor" our customers server.
I search a solution with central management console and remote agent that will be widely installed. I should be free or a cheap (max 2000€ one shoot for 100/150 servers)
We already test some solution (Pandora FMS, Nagios) but the problem is that the communication is made from server to client and not from client to server.
This mean that we have to open port on router or install some kind of "proxy" on remote location.
WE DON'T want that and we still searching for a solution where that is the client who initiate the communication to the servers (htts, ssh or something else)

We need to monitor simple things like:
HDD free space
CPU CHarge
Free memory
TCP listening port
Event Log reading (wmi)

We would be happy if it could check some more things:
hdd smart monitoring
specific service check (hyper-v, DNS, Remote Desktop Service,...)

We prefer not us this technology:
snmp (all nodes can diagnose himself without it and report himself its state)
server to client communication (involving opening port on client side)
server to proxy to client

So with all this informations, does somone know a good solution free or paid?

Thanks for help!