Enormous effort is required to deploy, configure, and maintain a system that monitors your company's IT infrastructure. The configuration for several hundred machines can take months. The effort required will also depend upon the scope of hosts and services that should be tracked—the more precise the checks need to be, the more the time needed to set these up.
If your company plans to monitor a wide range of hosts and services, you should consider setting up a machine dedicated to Nagios that will only take care of this single job. Even though a small Nagios installation consumes little resources, as it grows, Nagios will start using more resources. If you set it to run on the same machine as business-critical applications, it can lead to problems. Therefore, it is always best to set up a dedicated Nagios box, even if this is on a slower machine, right from the beginning.