Custom Drupal Register Data Module

I am building a "register anything you like" site. These are the classes of users: admin, non-registered, retail-user, certified-user. Certified users can be a police department, government agency, or even a corporation. We need a Custom Module in Drupal 5.1 so a retail-user can store registration information on any item he chooses. Ability for admin to pre-populate fields; retail-user can also generate custom fields for his own items. Ability to tag the item, so that all his items with this tag can be searched for by himself, or maybe others. Able to allow specific classes of other users to search and find his item, such as a police department. Example: The camera is stolen, user wants police departments that are registered users of our site to be able to search and find him because he has registered his camera on our site. By registering his camera on our site before he lost his camera he has the serial number and other info for police and insurance purposes handy. Example: User wants to save registration information for a camera he purchases. His records contain the serial number, when purchased, how much money paid, if extended warranty purchased and terms of the warranty, etc... Later, user wants police departments to be able to search this record, because the item was lost/stolen. The serial number can then be searched, or even the "camera" tag can be searched by any police department that has registered with our service. A police department can search and find his entry, but a retail-user with a camera might not be able to search and find his entry. A log of revisions is kept for the item maybe. Second Example: same as above, but item was not lost nor stolen, retail-user does not want anybody to be able to see the record of his camera, data is for retail-user's view only. Third Example: His grey cat with yellow eyes is lost. The cat is registered on our site by the retail-users. Anybody, even non-registered users can search for cats, grey , california, yellow eyes, on our site. Somebody finds the cat and he goes to our site. Cat owner is found using our site, everybody is happy. Simple customize-able blocks for the users and administrators.