Architecture for a user content driven site in MySQL/PHP

I am trying to create a site that would kind of act as a “classifieds” for my school. I like the idea of wordpress and I would like to use it, but I’m running into a few architectural issues where I think I could use some wordpress community feedback.

For example, if I script my own login page, how do I create a PHP session when a user is logged in (on top of the wordpress php session)? Are there existing plugins/hacks that would allow me to easily create a user login/registration/profile page without using the wordpress interface? Using these plugins can users create posts in a format that i create (picture, title, post, contact info, address etc.) ?

I believe that anything is possible, although at this point I am really just trying to figure out, at a high level, which direction will give me the least resistance while still presenting my users a professional and entirely customizable interface.