It's built for the lowest common denominator. Literally all you need is PHP 5.4+ and Mysql 5+, and curl. There aren't any required modules for basic usage but you may want smtp (if you don't want to use built in php mail function). Cron for recurring tasks, which isn't required for basic usage, but for things like recurring invoicing and reply by email. I'll add a new page in a couple of minutes that lists all the details.

Also agree about a validator script. I'll work on that too.