The basic models (usually <$100) pretty much have a manual mode only that you program in the desired shut down time. The more advanced models are the auto-timers, which monitor some parameter(s) to determine how long the motor should idle to cool down (they also have a manual mode). The advanced models also usually have other features (my HKS Type I can measure 0-400m, 0-1000m, 0-100km/h, as well as act as a stop watch and I think a lap timer) which you may or may not use very often. I was able to mount mine in the clock location with very little modification - I had to remove <1/8" of plastic around the corners. I used some metal strapping that I found at Autozone near the exhaust repair supplies. I cut a notch in the strap so the wiring harness could go straight out the back of the unit without getting pinched, and wrapped the contact area w/ several layers of electrical tape to keep the wires from fraying. Here are a couple pics of the install:
