Exchange Server replacement

Exchange Server replacement

Our current network layout is as follows:
1 Active Directory server (WinSrv 2008 R2)
1 MS SQL server (WinSrv 2008 R2)
1 remote web server where SmarterMail Pro is handling all the mail related
tasks
6 POP3 accounts with local MS Outlook 2007/2013
We'd like to centralize mail database backup, use a single Active
Directory login and share calendars, contacts, etc.
Please consider that:
We plan to reach 10 accounts in a few years
We have a static IP but no knowledge at all on how to keep a local web
server safe and with a 99.999% uptime
A limited budget, of course.
What do you recommend?
Thanks.