Connecting multiple lans while sharing internet connections

Hi,

I would like to connect 4 locations. 2 of them already have internet. The internet I would like to share as well as some file, web, camera servers, ...

I have no idea what kind of firewalls I need and I don't know how to setup the security between the different locations.

Most of the cabeling is already there and I have a lot of wireless access points and unmanaged gigabit switches that I can reuse (the're new) But I have a small budget for buying extra firewalls.

Your comments are much appreciated!

This is what I had in mind: