If all you want is to produce okay spur gears, get a wire EDM, it's very versatile. If you want a gear machine and for students to understand gears better, I would skip anything complicated or CNC and buy a shaper. It's one of the best options for understanding how gears are generated, so the students will learn more than they will programming an arbitrary shape on an EDM. In my experience teaching, gear shaping is the process that they 'get' the most. It's also versatile, can do ID work, cut up to a shoulder, etc.