I ran a cheap Walmart vest for years. I picked up a Marsupial ($$) a couple years ago and am trying to maximize it's versatility. My dog drinks a lot of water and I carry first aid for both of us so a bladder and 2 water bottle carriers is appreciated. Yes, I can do all of this with a small daypack and a bird "stringer." One benefit of the bird harness set up is the box pouch/bird carrier in the back can also double as an emergency litter for my 60-lb dog. I also swap out hip belt pouches and replace the box pouch/bird carrier with a large pouch for scouting, archery, and turkey season.
All of that to say: There is no need to pay a lot for any of this stuff. Start with something. Figure out what you like and don't like. And then modify to your needs.
