Exactly the same licence is required to fly freight. It's exactly the same airframe so therefore there isn't a different licence.
Flying freight is a job. Yes, it is often used as a stepping stone to other things but you're an employed professional pilot. Therefore, freight is "commercial".
Getting a freight job is just as hard as a passenger flying job, market forces etc. Chances are you'll have to buy a rating for a turboprop freight job (eg Emerald) but from what I can gather, they want a min of 1000 hours to get a look in.
Have a look at ppjn for more info regarding T&C's