Please i really need your help sensor and bot crea

i am a first year computer engineer student and i just searched the net and found audrino and i just love it! ive spent about 1 week researching lots 4 hours daily for past 1 week and i am now thinking that tomowrrow ill order the basic starter kit. I want to make a sentry gun which jus turns on this pen laser and finds a moving target and points it at the target. So ive learned i need a servo motor which i will go out and buy. Ive spent numerous hours researching different methods of detecting target such as using webcam to either Video manipulate, Track, or object recognition. They were all a little useless for me because they require expensive equipment and extreme levels of knowledge which i am trying to obtain. But, ive found this one source code which basically uses a Devantech SRFO4 Ultra sonic Range Module and utuba standard servo to make the target detection and servo move. But the thing is i already have a tight budget because university and books are expensive and spending 45$ on the basic kit is quite a lot to me i dont want to spent another 30$ on this sensor...there is some sensor for 8$ HC-SR04 Ultrasonic Sensor Distance Measuring Module but i dont know how to use it or if it works or anything so what can you suggest or what could i do? i REALLY wanna make this sentry badly ive spent hours searching and im willing to go a hell lot more to obtain this goal because of how much it excites me. If you think i should buy the sensor then will it be useful in the future for my field as a computer engineer or will i ever need it and will the price really pay off? is there any alternative? help me out.... :-[

sometimes people use hacked pyroelectric sensors (aka "motion sensors") for this kind of thing. Doing computer vision on an arduino would be tough.