The free version's license is for personal, non-networked use only.
As for "economical," these are servers. My advice would be to ignore economy, and focus on what will do the best job.
An ounce of prevention... yeah.

Don't forget to sweat the details on the configuration: no files exempt, use heuristics, scan within compressed files, and never make it ask the user for confirmation before taking action. We use McAfee VirusScan Enterprise where I work and it is pretty strong stuff.
http://www.mcafeeb2b.com .
Also, this would be a better question for the Operating Systems/Software forum, remember that they do have one here
