I would like to ask for your help in setting up the proxy server to protect and to monitor multiple servers of my company

I work as Network Engineer. I want the servers to be stable, and if a server is down, I will be notified accordingly instead of that customers call and tell us;

I wish to know if someone have any experience, ideas that you can share, and any available software or programs that I can use. I ran through the information of ISA server, provided by
Microsoft. ISA can provide us with various number of functions, including monitoring multiple servers, alerting when the server is down, setting up firewalls, access-control list... Moreover, the servers are running on Windows OS, and I think that ISA support Windows (Squid supports Unix OS). However, there is one problem is that ISA is expensive (1500 USD). Thus, I am still considering if
I should recommend ISA to my company.
So, at the moment what software and programs are you using to do the job?
Thanks very much for your reply