Start with what/how horses have evolved to eat & go from there; They are 'trickle feeders' who's digestion is designed for near constant, tiny amounts of low grade(compared to cattle fattening pasture) roughage. High fibre, low starch/sugar. So grass/hay is about the best staple.
If they need more energy/nutrition, it should be fed little & often & is best being low starch/NSC. Cereal grains are high starch, some more than others, and while it may be necessary & suitable for a horse that requires extra energy, generally these days there are better, safer alternatives, such as beet pulp, for eg.
Nutrition-wise, horses have evolved to roam large territories, browsing a variety of plants as well as grazing, from different soil types. So our domestic horses, stuck in a small paddock, grazing a limited amount of grass species aren't likely to have well balanced nutrition. Therefore a good quality complete supplement/ration balancer(in this I think you tend to get what you pay for & the most expensive may actually be more economical, as you feed way less, as well as being best)
Safergrass.org and feedXL.com are great sources of info/help on diet and nutrition. Hoofrehab.com also has a fair few articles/links on diet too, as it relates to hoof health.