First, Kudos to you for thinking ahead on what a proper feeding regimen should be
That's the easy part because it can vary from horse-to-horse
1. Type of living environment (lots of pasture to run and graze, on a dry lot with no grass, in a stall much of the time, etc.)
2. Type of job the horse will have.
My horses don't get ridden anymore and are on a diet based on my two horses with metabolic issues. They all eat the same base feed because it's healthier for them; the variation starts when I have to add in to accommodate insulin issues and arthritis.
Therefore, it's much easier for me to hand mix everyone's feed; they eat the same but different
If you only plan on doing light work with the horse such as mild trail riding or showing in pleasure classes as opposed to hunter/jumper, reining, etc where the training and classes are intense, your future horse should be able to do well on nothing more than a grain-free ration balancer.
Ration balancers are NOT meant to be fed with bagged feed as they already have vitamins and minerals in them.
They are meant for horses who eat hay and/or grass.
Nutrena makes Empower, Purina makes Enrich 32, Triple Crown makes Safe Starch, just to name some of the top brands.
Don't feed your horse "cheap" just to save money up front on a bag of feed; in the long term it may very well cost you in terms of the horse's health.
Hope this helps some; I'm sure others will come in with their thoughts