1 for BP you would need a wooded storage box built to a specification laid down in the regulations (they can be brought for about £50). You would need to have this cleared by your FLO.
You would need an explosives licence and another for transportation. A lot of fuss when getting started.
Alternativly you can use BP equivilents such as Pyrodex or 777. There is no difficulty buying and storage is up to you. (I buy through the post and mine sits in my gun cabinet).
2 You should get close on 1000 shots from a bottle of powder at around £30
100 caps is around £3.50
Wads are a bit pricey at about £3.50 a hundred but you can make your own from wool based felt.
Balls are around £6.50 a hundred. Again you can make your own. A mould is about £20.
3 Same as any other FAC, make sure your club caters for m/l and that you have suitable cabinet.
Go for it, its good fun.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane