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!
🙂