I need to set up a reverse proxy where the backend web server is serving https pages only. I've never set up a reverse proxy and would like a brief overview of how it works.

One of my questions is whether the proxy needs to run https also, or does simple http suffice?

Second question would be whether to use Apache, varnish, nginx, or squid. This is for an internal site for a small company, so not a lot of traffic expected. Maybe a few dozen users each day.