If I were You, I'd try all that on regular PC first, starting only with video processing using web-cam and some quick prototyping language, if that works and You can get positional data, I'd add 8-bit microcontroller and small servos mounted by laser pointer (or something similar) to fine tune the algorithm, and only then I'd go for Raspberry PI, get it configured and algorithm rewritten to whatever language Raspberry PI uses to do image processing, and full strength servos man enough to handle spotlight.
There are probably other ways to achieve same thing. Give it some time and other forum members, most likely, will share their knowledge regarding the matter