Hi,
to host the PAC file no performance is required. The server will need a tiny web server only hosting one small file and deliver it a few hundret times a day. The minimum you outlined is fine. If you check the network throughput, memory and CPU utilization and see no bottleneck, all should be good.
The key is if the web server installed is capable of serving the request/second. I would assume that every IIS, Apache or Nginx is optimized enough to do this job.
In doubt, you can also host the Proxy.pac on the Web Gateway. It can be created within the rule engine and served by mwg-core, do you can even utilize HA functionality and mwg-core is known to handle a lot of requests/second.
Andre