Sounds like a neat project and one definitely arduino-worthy. It really depends on how much precision you want in your project. If you just want to detect people you can use a PIR sensor(usu $10-20); if you want to know how far away they are then an ultrasonic sensor is the way to go. there is plenty of simple documentation for the parallax PING ultrasonic sensor which only costs $30. Depending on your budget, you can have all the professional data that the i Cube x sensors provides but it'll cost you $$$.