If eat only rice with lentils and beans..and onions or carrots..how healthy would be after 4 months?

There is plenty of protein in that selection coming from the beans and pulses, lentils and chick peas are especially good. I would add more fruit and vegetables, apples, pears and citrus fruits are usually the cheapest. Cabbage and root vegetables including potatoes are the least expensive, try the markets to save money. You could add bread, pasta and cereal for fibre and carbs. A vegetarian/vegan diet is usually less expensive and much healthier.