multiple server monitoring

We support approximately 20 Drayteks (small ADSL firewall/router/VPN appliance), 10 Linux servers and about 5 Windows 2003 SBS servers. Servers are all at different customer offices, used for file server, web proxy, email, fax server, database, etc We need to install a monitoring server so that it automatically checks backups, free disk space, cpu load, services, etc at regular intervals and alerts us when a problem occurs. We can access most of the Linux servers by SSH, we usually connect to VPN and use RDP for the Windows servers. I suppose we will have to use Telnet or something to access the Windows servers. I'd prefer the monitor to run on Linux, but don't mind if it has to be Windows. I'd prefer open source software but would consider an affordable commercial system. The job is to advise us on which monitor system to run, setup the monitor server, setup any client software that is required on the customer servers and train us how to setup client software on future servers and to use it.