I agree that a server might not be the right thing for you. You can run an email client on OS X and connect all of your existing email addresses to it. Same goes for calendar and iMessages. File sharing is very easy to turn on. Simply go to System Preferences, Sharing, File Sharing and specify the folders to share and which users have access.
If you were trying to host a private email server and already had purchased a domain name (myname@mydomain.com), a private calendar server, and a private iMessages server for use only within your home then OS X Server might work for you. Keep in mind that you would most likely need a business account with a static ip address from you internet provider because most ISPs block incoming business ports like web, email and calendar. After that you'll have to set your new iMac up on a DMZ and port forward the necessary services in your router.
If that hasn't scared you away yet then you must be an adventurous techie person. Give it a shot and see how it goes. If nothing else it will be a good learning experience...
The last thing I'll mention is that it's never a good idea to use your primary computer as a server. If anything happens to your computer (it stops working, slows down for one reason or another, gets a virus, you start messing with settings, etc) your server goes down too. The question to ask is what impact would a complete loss of file sharing, email, calendar and iMessages have on you and your home?