Your $300 budget can get you a 20-30cc sized 3d plane but it will not cover a powerplant, receiver, or servos. The weight of the GoPro would be an issue for such a small plane if you want acrobatic ability. Stepping up to a larger plane would help but would destroy your budget and be a lot of plane for a beginner. Upping your budget and/or reducing the camera to a keyfob camera would help a lot.