if i can just join in on this one, i own a theoben rapid 7 air rifle, have been shooting now for the best part of 22 years, most fridays 3-4 of us meet up at a farm to help with rabbit control.
The rifles we use range from 45 ft/lbs right up to 65 ft/lbs and there all air rifles, they are in various calibres from .177 through to .25.
If you are going to do this professionally, then get the right rifle for the job, as the others have said, there is nothing worse than hearing a wounded rabbit scream, we always try to go for head shots, and if we cant get it then we wont take the shot.
As for butchers, thet are not really bothered about taking them from you, however the farmer might for his dogs, ours does and i have a few for my 2.
how are you going to get round the fields? if walking then you got to think about carring stuff, are you going to do it at night? if so your going to need a red lamp, its going to cost you more than 100 quid to do it right.
if you have not shot before i suggest you target practise till you are confident with the gun and things like acting against you like wind.
sorry for the long reply but if you are wanting to enjoy the sport, set yourself up right from the start, and welcome to the world of field shooting
richie