Ensuring that a sever and website are secure

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My company is looking into developing web-based data hosting and access
system for several of our clients.  Their data is somewhat sensitive,
and they are very concerned about security.  We are not an IT company,
and do not have experience setting up secure servers or websites, but
we would like to be as sure as possible that the servers and data on
them are secure before we launch this service.  We need to know who to
go to both for help securing our sever and website, as well as for
testing whether it is secure once the process is complete.

Several people have recommended having a security audit done once our
technical staff believe the website and servers are secure.  Is this a
good idea, or a waste of time?  If it's a good idea, who are the best,
as well as the most respected, companies that do this?

Any advice and suggestions will be appreciated.

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