Air/fuel ratio moniter question

If it doesn't datalog, its useless. Look to see what features it has.

The A/F gauges that use the O2 sensor signal will jump all over the place. The reason is that the O2 sensors "switch" between .2 volt lean and .6 volt rich with a curve that looks like the drop off a high cliff. The curve is almost straight up and down, so the voltage shoots from .2 to .6 and back down . again 2 or more times a second at cruse. You won't get much useful information except when the mixture is extremely lean or extremely rich, there is no middle ground.

What you need, ideally, is a sensor package that's designed to work with a wide-band sensor (an ADDITIONAL sensor you'll add to the exhaust system), and a package that lets you see instant readings while operating the car (gauge, LED, etc) as well as the ability to datalog the results so you can review a longer period of time after you've driven the car.

You're not going to get that for $200. Maybe 3 years from now. But not today.