Server-Side Caching for WordPress Sites

I’ve been doing some research into caching solutions for WordPress sites running on a LAMP server. I’ve looked into plugin solutions such as WP Super Cache and W3 Total Cache, and I understand the performance gains here come almost exclusively from serving up static HTML directly from the server’s local disc. I’m worried that implementing a solution like this into my development process may cause some issues with those parts of the site that require server-side scripting to function.

After looking into Varnish, it seems a reverse proxy cache like this really shines in increasing the number of requests per second and may alleviate me from my worry mentioned above. However, most of the configurations I find for it seem to be for Nginx. It would be ideal to avoid switching from Apache to Nginx or switching MySQL to an alternative database solution as I would like to offer caching solutions to my existing clients without having to do an overhaul on their server/database setup.

The fact that Varnish has its own configuration language, a 300+ page manual, and requires different VCL setups for WordPress vs. sites using another CMS is also bit off-putting. It seems to be a steep learning curve, and I’m not sure if the ROI would be worth the effort. I may defer to Pantheon for my future projects, as their WordPress installations come with Varnish by default.

Here are my questions:

1. How much traffic should a site get to really leverage the capabilities of Varnish as opposed to a solution like WP Super Cache or W3 Total Cache?
2. Are there other reverse proxy caching solutions for Apache with a simpler implementation process than Varnish?
3. Do you have any resources you could recommend for implementing Varnish with Apache?

Many thanks in advance.