What Should I Know Before Buying My Own Dog?

Okay, so I am a teenager, old enough to get a job, and I am very responsible and my mother agrees that if I pay for my own dog I can do it – but I have to pay for the vet bills, the toys, food, and anything else that comes along with it. However, she said if the dog gets really sick, she will pay for any surgeries that may need to be performed. I do not plan on getting pet insurance, btw.
I was just wondering what initial costs there will be besided the actually price of the dog… what essentials will I need to make sure he lives as happily as possible?
I would like to adopt from the pound, however most dogs there have been abused and since this is my first dog I’m worried that I won’t properly be able to help/train it. On that note, I DO NOT want a purebread due to the health problems they are prone to get – so what cross breeds are good? I was thinking Cockapoos because my friend had one and he was an amazing dog… Also, I do want a small – medium sized dog, no huge ones.
I live in a small town so there’s lots of open space and I have a big backyard so excersize won’t be an issue. Also, I would prefer a breed that’s good with cats, children, easy to train and isn’t very talkitive 🙂
Thanks so much!

Why Would My Dog Suddenly Try To Attack Another Dog When He Has Never Done That Before?

We were at Petco with our dogs (6.5 mth olds Pepper-cockapoo, and Bandit-lab/pit mix) walking around enjoying ourselves when we go to turn down an isle and there is a man with a full blooded brindle pit bull and a smaller baby pit bull in a buggy. The man’s pit bull sees and and tries to head down the isle towards us. He wasn’t growling or barking or anything. He was just pulling his master trying to get down the isle.
Bandit (lab/pit mix) suddenly puts the hair up on his back and starts growling and showing his teeth. He was very agressive and he isn’t like that. We have been to Petco before with him. He takes training there every Tuesday. There is even an agressive Rotweiller in our class and he isn’t even that way with him. He has been around other dogs outside of Petco. We have an older pit bull down the road, 2 austrailian shepherds, and a border collie, and he has never been agressive with any of them.
He has been food agressive at home, but only with Pepper, never us. But then he and Pepper will share the same water bowl at the same time and he doesn’t have any problem with that.
We immediately pulled him away from the situation and scolded him. I have never seen him act that way to another dog. So does anyone have any ideas what could have been going through his little (well, not so little since he does weigh 45 lbs) mind when that happened? I would appreciate any help or suggestions.