What server OS do you run?
i reccomend Windows Server 2008, and you should look into vpn networking for your offsite computers. Server 2008 supports this though you'll need to look at what routers/ISPs will give you the best performance as vpn's can be a bit slow.
Here's a little info on vpn networks: http://en.wikipedia.org/wiki/Virtual_Private_Network
Depending on what kind of work they do (office, etc) i reccomend sharepoint server (if your looking to drop money on it) it's a very nice enhancement to office.
if you want to go the whole linux route look up debain, during the install you can set up a lot of server software and it will give you a nice set up. thought i'm more of a windows guy when it comes to servers.
you can also set up roles in server 08 like dhcp, dns, and domain services. i reccomend them though domain/dns can be difficult if you're a newbe.
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To you additional detials... VPN is free and can be set up on any windows installation. What windows do you have?
set up server2008 vpn server:
http://www.windowsecurity.com/articles/Configuring-Windows-Server-2008-Remote-Access-SSL-VPN-Server-Part1.html
http://www.windowsecurity.com/articles/configuring-windows-server-2008-remote-access-ssl-vpn-server-part2.html
and if you're not using a server operating system windows 7 can still act as a vpn server: http://forums.techarena.in/tips-tweaks/1143104.htm
and so can vista and windows xp
set up client on windows 7: http://www.tomshardware.com/forum/138-63-windows-client
vpn is free to set up though in a corporate environment you'll want to make sure you have a good router or it'll be really slow.
for your firewall software you can use zone alarm, it's supposed to be good.
Hope i've helped.
~Pyros~