I would go with .2s for sure. Go ahead and get the crosman 5,000 for $10, they aren't the best but they won't cause any damage. And dirt cheap. They won't give you as much accuracy as say TSD .23's or TSD .2 for that matter, but they will be FAR above .12's.
So in short, Crosman .2's are as cheap as I would go without causing damage and/or or horrid accuracy. Used them in my CA m15a4 which is still going strong 20,000 + rounds later.