question for overhead for having webserver facing public domain

[Yasushi Okubo]

Hi, experts

We have a new project to place one of our linux servers outside our 
private network to provide public access to our web-based application.
But I do not have an experience for security management to protect this 
box from security threat.

So, I would like to have advices for the following issues [I am planning 
to install AS4.0 onto this machine]:

1. any good security management software tools ?
2. other concerns by owning a server in public domain beside security
3. how many hours per day sys admin should expect to spend to mange such 
a machine
4. any web resources to manage such a machine

Thank you,
yasushi