Point Awarding System

I'm designing a website that's going to be the companion of a tv program where music videos are going to be presented. Kinda like MTV when it was really about the music. So our target market are going to be mostly teenagers. But the idea is to give the website a life of it's own, so we want to develop or to have someone develop a point awarding system for our registered users. The users will be awarded a predetermined amount of points first for just signing in then for different activities like answering polls, voting for their favorite videos, posting questions, etc. Then the users can exchange their points for prizes. Of course the points have to be substracted or added automatically depending on the activity. I want the user to logon to a control panel where he/she can view and manage his/her points. And i also want to have an Admin area where i can view everybody's points and add or subtract points from any member. On signup I want error checking on email address so that email addresses are never allowed to register twice, also a lost password retrieval feature and modify account. And finally, NO user can for example click on a link or vote for a video more than once in any given day. If they do they are not awarded more points because they were given the points already, I am sure you know what I mean. Maybe you already have this covered but just to be sure, i want to be able to add or delete the features of the site (polls, voting areas, links) where the user is going to receive points. For example, i have this new poll, and through the admin area i want to add this poll to the point awarding system and i want to assign it 500 points. (the user is going to be awarded 500 points for participating in the poll). The goal is to be able to customize this myself and not to have to contact you for every little thing. Obviously I don't know what you have in mind in order to create the script, but from what i've researched one way to create it is using "session variables". i know they're not recommended by some programmers cause they tend to put a heavy load on the server when a large number of users are visiting the site at the same time. So i want this script developed with PERFORMANCE in mind, would it be using "session variables" or another approach. So please explain the pros and cons of the system you would develop. Definitely we're gonna need addons or modifications to the system in the future. There's no need for any graphic or web design, i need just the actual system/script. Please be able to complete the project in a maximum of 2 weeks. Payment will be made via PayPal or through the escrow system, ONLY after total completion of the project. Thanks to all.