You've obviously done your research and you're off to a great start!
1 protein source means one type of animal. Chicken backs, necks, etc., are still a single protein source.
You don't have to do the organs every day. The percentages given are over time. I usually try to balance mine out over a week or so.
If you can budget $18/week, that gives you $2.57/day to spend on 1.3 lbs of meat. That's $1.97 a pound. Easy peasy with chicken. When you can score some chicken on sale at say 99 cents/lb, that frees up your budget for a higher per lb price on beef.
When you start adding in the other protein sources, such as beef, you'll want to look into buying in bulk and freezing, or other sources. There are lots of good tips out there. Never be afraid to ask butchers for free meat. Also if anyone you knows hunts or fishes, ask them too.
The only "supplements" I add are canned sardines. The dogs add a little lawn grass when they feel like grazing