What is mostly data? You mean the site is mostly static content and you wanted to keep it all the same level of simplicity? I've found making websites where one can create a username and password to be way simpler than implementing design-by-committee systems, so I don't really understand why that wasn't the obvious choice.
It’s simpler to create a db, password, keep up with correct salted hash scheme, email password reset, mail service, getting through spam filters, user profile icon? As soon as you’re dealing with customer info you took on a bunch of requirements.