Wordpress is great for editorial purpose , drafts, reviews , auto-save, new roles system and other stuff ...
This is its domain , not other .
Wordpress Plugins System doesn't forward OOP approach ( i don't say there aren't plugins written with OOP approach , but in a complex scenario hooks system result very tedious)
First try to explain this to your customer .
Documentation , like other developers explain , isn't suitable , you can try to start with http://codex.wordpress.org , then the best move is to analyze directly portion of wordpress code interested to your modify and find all filters/action hooks , this is a complete db of all hooks ( http://adambrown.info/p/wp_hooks ) .
Finally pay attention to hooks that will become deprecated such as wp_authentication used in many plugins.
