InmateLifeskills.com is likely the most extensively-adapted WordPress site I’ve built to date. Years ago, I added checkout systems through both PayPal and Authorize.net, both using a good deal of PHP.
The site was optimized to target a very specific phrase, “Correspondence Courses for Inmates“. When originally optimized, the site ranked in the top 3 for that phrase. Today it still shows up in the top 10 with no additional promotional efforts. The site is currently hovering around 7th on Google’s natural listings.