That cool JC! I've got the same freezer and love it to pieces. Can get 8 cornies in there without a collar.
You have many options to expand. If one was so inclined (read insane) you could add load sensors to the bottom and weigh each keg, kind of like keg bot. You could also put a pressure transducer on the co2 line to display bottle pressure and serving pressure without having to open the lid. Flow meters on the serving lines, etc etc. Not that there is anything wrong with the Ranco/Johnson temp controllers, the Fermtroller gives the user the option to exercise some creativity.
Cost wise, I'd guess you're looking at around 100 bucks. the new dedicated fermtroller boards are going to be cheaper than the BrewTroller boards so I think $100 would be the upper end of the cost when they become available.