Start off by asking the seller what they have been feeding. This can be a good starting point, but doesn't necessarily mean the seller picked quality, nutritious feed. Ask your vet for his advice, but keep in mind that vets get very little training specifically on equine nutrition in vet school and not all have done additional study on it or kept up to date.
In general, you should ensure forage (either hay or pasture) is the basis of your horse's diet: at least 1.5% of his ideal body weight daily, but some horses need upwards of 2-3%. Whether you need to feed any hard feed at all depends on the horse and his workload. If not getting fresh grass, you'll need to supplement something with adequate levels of vitamin A, vitamin E, and omega-3 at the very least, since these degrade rapidly in hay.
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