Advice on setting up a site with front end registration in WordPress

It's few months already and you probably have this solved byw but I'll try to do a little necromancing resume here anyway. We don like unanswered questions right? ;)

There's a recent topic dealing with the front-end login & registration: Front-end Register Form

You can change what WordPress displays to the users based on their capabilites by using the current_user_can() function.

There's also a topic about loading alternate CSS:
How can I version the main CSS file?
Be sure to check related questions too.

By using body_class() or post_class() you can change the CSS class of html body (or post) depending on the logged user.

If those changes you want to achieve are intended to be more user based then role based then you can work with update_user_meta() and save some options in there.

None of the above is hard to achieve. All mentioned functions are well documented.

EDIT: Oh and the ticket system - You could use custom post types with front-end posting as a good starting point.