To the OP:
You can naturally carb in the keg and serve with the charger. Use your charger to seat the lid when you rack to the keg with your priming sugar, wait 2-3 weeks at room temp (at least), and then chill and serve with the charger. This'll work for a keg or two and then you'll get tired of it and get a tank and regulator. It is possible though, especially good if you're trying to piece your system together over time.