I've read through this thread a few times, and thought it was time I should try and give you some answers.

First up, a boxing gym doesn't cost as much as you'd think, and if you want to box in amateur matches you're going to need to join one asap. I'll try to answer the other questions as you posted them!

1:How long is a piece of string. You can be super fit and hit the floor in the first round or you can be a big tub of lard with a good jab and beat the hell out of someone, as I rule I'd suggest get as fit as you can.135-140 pounds puts you in Light weight/Light welter weight category.

2: Not as far as I'm aware, points are scored when judges agree that a punch has landed cleanly on any scoring area (front of head, upper body, lower body, L/R side). Body shots are avoided by many fighters as this leaves your head un-guarded and as you lean in you become vulnerable to a big hook to the head.

3:In the corner or on the ropes is a good place to start, or slip the opponents rear hand and follow with a hook to the body. If a sparring partner is throwing a lot of jabs and straights to my head I will often move in on them, opening my face up to land 2-3 quick hooks/uppercuts to the body. If you're going to try this you've got to know you can take a few hits to the face.

4:I watched superman, and that flying business seems quite easy! In all seriousness, don't focus on knock outs, its a dangerous way to fight for you and your opponent. Sometimes you will knock someone down, sometimes someone will knock you down. Box for points not injuries.

5: Join a gym. Hitting a bag trains you to hit a bag, not how to box. If you can't join a gym yet, focus on cardio work, get your legs and core strong, build up your calves with lots and lots of skipping and sprints and squats. That way when you do join a gym you can focus on skills rather than fitness.

Hope some of that helps