Choose the right hardware to build a squid proxy server (200 users)

I need to prepare a squid proxy server for my company. We have a new fiber connection (100 mbit) and I need to give access with user and pass, speed up navigation with caching and do a little web filtering. The only thing that I would really care about is correctly sizing the proxy's hardware because I do not want it to be our new connection's bottleneck.

I'd prefer to spend the lowest possible but I'd appreciate hints about what I really should get:

Ram? Cpu? Fast disk? Gigabit ethernet?

Thanks to anybody who'll help.