conditional registration/login

We need an expert to make an existing website application's access conditional to registration and login on our site. The registration and login feature is already crerated including redirection and user level assignment and needs to be configured only. The existing registration and login has also the necessary code to be inserted into the page which needs to be prodected. The user can browse our website for items displayed without registration but needs to be registered and logged in as soon he/she is ordering an item/information or wants to use our service and it is this feature mentioned above which we need to implement. The necessary messages (in a popup window) in informing the customer needs to be implemented. NO registration or login form needs to be created,neither any database mods. All scripts are PHP coded. Server: LAMP FTP access provided but no CP. Timeframe: A.S.A.P. Payment via escrow prefered.