Hi Bob,
The B-25 should be ok as long as all you are going to do is sport flying with it. Of course, i don't know for sure how much "stunting" it would do anyway! Try to keep it fairly light.
.012s seem to be legal now for a .25 so I would use them. But, it won't hurt to use .015s. '57 eye to eye or so is where I would start.
Bill <><