I think it's overkill for what you're trying to do.
The Ranco can be set to either heat or cool (electricity either on or off at a temp; it can't control the temp by itself...) I assume the Johnson is the same.
You're trying to keep from cooling the beer too much from the outside. Yet the thermowell is in the center of the carboy. By the time the cool has hit the center of the carboy, you've already got a few degrees difference. Since it's an exothermic reaction, it's producing heat by itself. I'd just use painter's tape to affix the temp probe to the outside of the carboy--it will probably be more accurate than the center and you'll have fewer things in contact with the beer.
On the flip side, if you're trying to cool it down in the summer and connect the controller to your refrigerator, the thermowell in the center makes more sense.
If you are planning for the future, then great--you'll want this kit and a refrigerator. If you are only worried about keeping it warm, then it's overkill. I have the same brew heater as your link and like it a lot. Too bad they don't have a dual stage Ranco/Johnson as an option--then you'd be able to control for cool nights/warm days like we have here in San Diego that sometimes exceed the +/- 2 degF settings I use.
