Just having a quick look with google, digikey do a photoreflective infra red photointerrupter, with a logic output you could connect stright to a uC, all you need is a piece of reflective tape on your wheel, or even just a blob of white paint.
The best kind of 'box to use for this application would be a worm drive, as when stopped the load cannot turn the motor, but I spose it depends on your application.
You wont get really good accuracy unless you implement a full servo, but as you said you want simple so try what you have.
If you use a fet you could implement a brake function, use the fet to put an effective short accross the motor once you have turned itv off, this will stop the motor from running on as much.