Read part 61. You don't need 250 hours in an airplane, you need 250 hours as PIC in an airplane. You could go straight to commercial add-on, but you need 50 hours PIC for that which would need to be solo. The cheapest option would be to get a private (20 hours dual) then count everything else as PIC. You'll need 270 hours airplane to get ATP. Do you need ATP or would a CPL suffice?