Filtering firewalls don't require fancy hardware. They are little more then simple routers.
All you need is:
With some systems by using a serial port console, you can even eliminate the monitor and keyboard.
If you need a proxy server that will handle lots of traffic, you should get the largest system you can afford. This is because for every user that connects to the system it will be creating another process. If you will have 50 or more concurrent users I'm guessing you will need: