Does your kegerator come with all the fittings?
If so, then it will probably come with a Sankey "D" coupler which fits most commercial kegs. If you want to switch over to a Cornelius keg, you'll have the buy the keg (about $40), fittings for the gas and liquid side ($6 x 2) and possibly some other fittings and tubing to connect to your gas system and tap.
In contrast, the deposit on a commercial keg is about $30 depending upon where you live.
Now I don't really condone destroying commercial kegs to turn them into brewpots or fermenters since they are technically the brewery's property and they cost them about $200 to buy. However, refilling it once or twice before returning it seems fine to me.
You can find a few tutorials on how to refill commercial Sankey kegs, but the basic procedure is to remove a clip-ring and then the center stem pulls out. If you've got the right tools (I've found that the thin can opener on certain models of Leatherman tools work well) it takes a few seconds to open it up. Without the right tools, it'll be amazingly frustrating. I don't think it's really a question of difficulty as it is having the right tool for the job.
Also take note that commercial kegs are 15.5 gallons for a half-barrel and 7.25 for a quarter-barrel (aka, half keg or pony keg). The typical volume for a batch of homebrew is 5 gallons. A Cornelius keg holds 5 gallons, which is what makes them convenient. Cornys are also smaller in diameter so you can fit more of them in conversion kegerators (using an old refrigerator).
Lastly, it sounds like you're thinking about fermenting inside of a keg. This can be done, but you need to vent the CO2 gas produced during fermentation through an airlock. There are a few kits available to connect an airlock to a keg.
My advice to anyone starting out homebrewing is to go to your local bookstore and get Charlie Papazian's The Complete Joy of Homebrewing. It's the best $14.99 you'll ever spend in the hobby and it contains more than enough knowledge for you to jump right in the deep end.
