Help choose OS for proxy server

[HaloAddict19]

I recently recieved a PC that a family member's co-worker was going to throw away. It has an AMD athlon system in which the RAM has been removed. I am planning on using the computer as a proxy server to bypass my school's content filter (which is rather ridiculous in how much it blocks). I was hoping to keep the project at a bare minimum of expense, I really only want to spend money on RAM, so I would like to find a freeware OS that can do the job, but I've seldom worked with any OS other than windows. If anyone knows an OS that would work well for my intended purpose and how to setup the proxy server please let me know.

[glc]

I am planning on using the computer as a proxy server to bypass my school's content filter

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