If not for you then for future readers of this thread, I'll share my preference for adopting from shelters or rescue groups. The advantage of an older dog, even if only a little bit older, is that you know exactly what you're getting - kind of the opposite of your perspective. Most shelters do temperament testing and won't adopt out a dog, especially with a 2.5 year old in the house, if there's any significant behaviour issues. And what behaviour issues the dog does have will likely be clearly spelled out for you. (BTW, I also love pitbulls, and the 2 that I've had in the past would absolutely have been as good with kids as any golden retriever or lab out there - though I don't expect you to take my word for that!) And as first time dog owners (or is that just as adults?) there's more opportunity for you to screw up the training of a dog. Lots of young adult dogs in shelters that got too big and unmanageable because they weren't trained, you know? I have one that is truly screwed up by her early life, whose owners didn't even have the decency to take her to a shelter, just abandoned her at the side of the road when she was 6 months old. So I know what can happen with an inexperienced owner and puppies.
Also, I second the plug for breed specific rescues. My parents have had two dogs from Golden Rescue Mission, and they've both been wonderful.
That being said, I see nothing wrong with getting a puppy from a reputable breeder, as long as you really do your research. Do they claim, for example, that their dogs have titles, and if so, have you checked their CKC or AKC registration, etc. That is a pretty good indication that they follow breed standards. Do they have both "mom" and "dad" on site? Are you able to go visit them and see their whole kennel set up? If you're not planning to show the dog, do they make you sign non-breeding agreements or insist on/arrange for the dog to be spayed or neutered? You can probably find a good list of questions and criteria online by googling something like "How do I find a reputable breeder?"
But anyway! Like I said, that was all almost more directed at anyone else who might be reading this thread now or in the future. Sounds like you've pretty much decided on a plan, and just really want to know about food. That's cool.
My first suggestion is to absolutely get the recommended food at first. Puppies are more fragile than adult dogs, and the change of environment, coupled with a change in food at the same time, can cause really upset tummies. Diarrhea and lack of appetite can put a puppy at risk of dying in fairly short order, if not addressed quickly. So buy at least one bag of the recommended food, and follow guidelines from your vet (or easily found online) about the speed to transition pup over to something more economical.
Now, high quality or low, bought at supermarket, pet supply store, or vet?
Though you can find any number of people who will swear by the cheap stuff, I have had the experience (more with cats - especially male cats, who can get blocked from cheap food) that you get what you pay for. I buy my food at the vet, and as much as people will argue that the vet has a motive to sell you that food, and talk about how little nutrition training a vet has, I will argue back that PetSmart (or Pet Valu or whatever store you shop at) also has a vested interest in selling you certain kinds of food, and that their sales staff certainly have less knowledge of nutrition than my vet. That being said, I asked my vet once, and he said that though there's nothing at the average supermarket, generally speaking, that he would recommend, but that if money was really tight he would suggest the Science Diet or Eukanuba that you buy at the pet store. In other words, there is some middle ground.