I make my own mix, but do it in the feed room. That way each horse can get exactly what they need without worrying about not enough or too much vit/minerals. I use alfalfa pellets, chopped pre-bloom alfalfa hay, whole oats, flax meal, and a vit/min supplement.
I only use oats on two of my horses that need the extra energy and calories. The rest of my horses are either too fat for oats or oats make them hyper. Most stock horses or similar do great on oats though, as long as you work them enough to burn off the starch/sugar in them.
If I were ordering a mix from my feed mill, I would probably go with 3 parts alfalfa pellets to 1 part whole or crimped oats (by weight). Then add my vit/min mix to each feed bucket in my feed room. Alfalfa pellets provide great calories, protein, and nutrients (including Lysine, an important amino acid). Oats provide more starch based energy, vitamin C, and some protein. Together, this should equal out to about 12-13% protein. You do not need any molasses, oil, or vitamin pellet/crumble. A dry mix will store better.
I would add the vit/min mix yourself, that way your easy keepers getting less feed will have just as much nutrition as your working horses that are getting more.
I use a custom vit/min mix from Uckele Equine. It costs me about $0.44 a day per-horse, which is as affordable as I can get it while still providing high levels of everything I want/need. I have them add 10 mg of biotin for hoof/coat, high levels of amino acids for muscle development/growth (we don't have much grazing), higher Vitamin E, copper, and zinc for overall health, no Vit D since I do 24/7 turnout in the sun, and no Iron since we use well water that is high in iron. I also have them add extra Magnesium for my fatties and Iodine as it seems to help with thrush and hoof health/growth. The rest of the mix is based on my local hay analysis.
If you want an off-the-shelf mix, I would recommend Uckele's Equi-Base Grass, horsetech.com's High Point Grass, Grand Vite, or SmartPak's SmartVite Grass. All of these are complete vit/min mixes for horses that get little to no foritified feed/grain.