Enhance a targeting system to the tank.
Use Mouse to aim, and the turret align to the mouse aim. (configurable max/min elevation, turn speed), simple graphics cross hair and circle will do.
something similar WOT/groundwar style.
Provider : A unity project, with sample vehicle for enhancement.
Deliverable : Working Unity project, c# source and graphics.
Budget : $50.
PS : if you are good with unity, it can be done in a couple of hours.