First thing: I would build a network with routers, and connect them with a technology available in your area. VPN over the Internet is good, and affordable. Good quality routers include LAN-to-LAN VPN capability, with little effort.
Next, you have a class C which gives you 254 usable addresses. Are less than 254 systems going to be connected to your network?
If so, then you can go ahead, use your class Class C, subnetting appropriately. (ref: http://www.learntosubnet.com)
If you are likely to have more than 254 systems on your network (include IP printers, etc.) then you should addresses designated for "private networks", such as:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
(ref: http://www.isi.edu/in-notes/rfc1918.txt)
If you use the Internet as your backbone (i.e., VPN) then you will need to do some address translation at the routers. Use Port Address Translation (PAT), sometimes known as Network Address Translation with "overloading". This will help you conserve your class C.
You will also want to subnet your class C to define the router ports that make up your WAN (VPN, Frame/Relay, ISDN, whatever) connections. Use a subnet mask of 255.255.255.252 which gives you 2 usable addresses (but uses 4 from your class C - you'll understand when you understand subnetting).
And finally, you'll want to reserve a few addresses from your class C to use on the inside of your Internet firewall. You will also do PAT there, and you may want to setup a few "tunnels" through your firewall for specific purposes like handling email, etc.
