Currently Being ModeratedMay 9, 2012 4:10 AM (in response to Ed_InNewJersey)
So each of your concerns:
Folder permissions: Apple uses the same Windows style ACLs so you can be as granular on OS X as you are on Windows.
Customer Database: What is this created in? If you say Access, then there will need to be a conversion. However, FileMaker is very capable (probably more so) and they are not offering free iOS clients. So there will likely be a conversion required but you can also gain much more function.
Connection to Internet: sounds like you are using the machine as a gateway. I would suggest getting a true firewall (for a company your size < $900) and then you need to do less on your server. Plus a firewall can also provide VPN and other services like content filtering, perimeter virus scanning, etc. Plus with a firewall appliance you can look to get a second line for failover and business continuity.
Exchange: Apple's mail service is not up to the task. If you must self host, look at Kerio Connect. This will give you Exchange emulation and great features such as spam filtering and built in backup. Or, if you are small enough, consider externally hosted mail. Rackspace will do Exchange accounts for $10/month/account. Do a three year cost analysis.
So nothing will move transparently. However, it all can be moved to Mac.
Remember, there are Apple consultants in your area (shameless plug) that can assist in these types of migrations.