1) Diet plays a big factor in it. Depending on how many hours a week you do it, you will get in shape. There is a reason professionals have "Camps". Where they go and do additional exercises, like running or swimming, where they work their core out, do bodyweights etc., BALANCE A HEALTHY DIET.
2) Again it depends what you put in it. You’re not gonna wake up one day and say 'WOW IM A BOXER, NO IM NOT GONNA GIVE YOU MY WALLET PUNK, CANT YOU TELL IM TOUGH". The change in your confidence happens over time, in fact you probably won’t notice it. You won’t become a killing machine, that’s fuckin gay, but a person who is confident in him/herself and knowing that they can DEFEND themselves isn’t.
3) No, YOU WILL NEVER BE A WORLD CHAMPION. There is literally nothing wrong with that. But you know what you can be. A student of the art .In the beginning you will learn to punch and move. Then you’ll start sparring. Keep in mind because you’re not going to the Olympics or going pro gyms have a "priority" over people who will. That’s just the way it is. But that doesn’t mean you still can’t learn. Go and watch YouTube videos on fighters whom style is best suited for you. Different body types use different styles. Your trainer will help you on that. Go read books on boxing. If your dedicated and once you go through some good old fashioned hard sparring/ smokers with good local fighters , you’ll decide if you want to compete or not. You want to build confidence that’s where it’s built. There is a difference between sparring and fighting. If you don’t wanna be a pro or a gold medalist, just do a couple of fights 2-5 is good. If you only have 1 fight and decided to stop, no big deal at least its life experience. 4) Gyms in NYC are fucking expensive man. For all sports, bjj, sambo, boxing, muay thai. All of them. BUT “Atlas Cops and Kids” in Flatbush is FREE. 0 DOLLARS A MONTH. But it’s a serious gym and you got to put in some fuckin work there to get noticed. You gotta pay with blood sweat and tears there. I know this is controversial but if you find a cheaper closer gym, I mean like a different martial art in general, for your needs, do muay thai. Only if it’s cheaper and closer. Striking Sports are just more mentally exhausting than grappling sports. Not to knock on BJJ, probably the best 1 on 1 martial art for self-defense, but my grandma could do it. Which is a good thing, because it shows it works for anyone. But my grandma couldn’t box, she could die in sparring and again sparring and fighting are two diffent things.