Whole foods are typically best. Less processing is better. Obviously theres always concerns about like good regulation keeping them safe, but generally produce when washed is fine. non-processed meats are better than processed meats. etc

If you want a really easy cheap filling meal that actually tastes good i will cook rice, and make a peanut butter sauce(Vinegar+peanutbutter+soy sauce) and mix that with whatever vegetables i have on hand. preferably the vinegar should be rice vinegar but thats not as cheap so just like a big jug of white vinegar will get the job done. Brown like some onions, peppers, carrots, etc in a pan with canola oil, then add in the peanut butter sauce ingredients and mix them in once veggies are browned. Then you add the cooked rice to that and just mix it up. You can add means and stuff too. The best cheap cuts of meat are pork loins (Can find half loins very cheaply atleast where i am. Like 7$ for a big chunk of meat that lasts awhile.) or chicken quarters. Chicken quarters are the entire leg+thigh together and can be found for like 89 cents a pound. Cheapest cut of chicken available. Just have some of that cooked up and add it to your rice. As long as you add plenty of veggies and stuff its very nutritionally balanced. Lots of fat, and protein makes it filling, and its pretty cheap too.

To save money on vegetables look for ones that are on sale, and buy in bulk then dice them up, and freeze them for later. I like to get tons of jalepenos, and carrots and freeze those cuz they're pretty cheap. I just have a couple containers i refill in the freezer with them in it. Same with herbs a small can of dried cilantro can be expensive but you can find a fresh bundle for 99 cents cut it up and freeze it. Way more for your money that way. Doing that with greens in general is a good way to add nutrients to your food. Just make sure you wash them before freezing them so you can toss the frozen ones right in the pan.