Junk food is typically quite low in fiber. It might satisfy you for a bit, but not long. Brown rice or legumes are high quality and inexpensive sources of fiber and protein, especially if you have the equipment to do them from dried in an electric pressure cooker (e.g. Instant Pot). You can also incorporate more fiber by introducing psyllium husk into your diet. Just be careful about psyllium husk and medication.
Fresh fresh fruits and veggies are always going to be ideal, but not always great if you're squeezed financially. The brassica family tends to be fairly cheap (cabbage, broccoli, boc choy, collard greens, mustard greens). Also check out Eat The Seasons, which estimates which foods are in season for you, and therefore most likely to be on sale.
It can be rough in terms of taste going from junk food to a healthier diet. Don't be afraid to lean into the herbs and spices.