No not at all! It really depends on where you are going and what you are going after. There is ammunition out there for all guns and configurations so you won’t need to buy a new gun at least to start with. For those of you out there with only 23/4” chambers id be looking to Bismuth or tungsten in appropriate shot size for the quarry you seek (see cartridge choice FAQ) if you are using the BASC permit scheme a friendly fowler may well lend you a more suitable gun for the flight.
There is definitely equipment that will make you more comfortable, waders, rucksacks, short length camo and water proof fowling jackets etc. but these are purely to make life easier as they are built specifically for the job however a warm water proof jacket, wellies and waterproof over trousers will suffice for lots of situations. Wait until you know the sport is for you before you spend any serious money.