Good for you. Things I should have looked into long before I did.
For horse nutrition, Dr. Eleanor Kellon is is in my opinion the very best and most reliable source of information. https://drkhorsesense.wordpress.com/
She also has numerous articles and publications on other sites including courses on equine nutrition.
Short answer: The effect of grasses on horses varies with geographical location and soil tests and with the breed of horse and with individual differences with in the breed.
Saddle: When I purchased my first saddle I thought of them as a back pack for a horse rather than a human. Nothing could be farther from the truth.
A saddle must distribute the weight of th rider as evenly as possible upon the horses back. If the average pressure on a horse's back exceeds a mere 1.5 pounds per square inch, blood circulation in the skin covering the horse's back ceases to flow. After two hours cellular death begins occurring. That is one of the source of saddle sores among others.
Most $5000 saddles are built upon a $300 saddle tree purchase by the saddle maker who is usually a gifted leather worker. But the horse's back does not see the top of the saddle, only the bottom.
The saddle must fit the width and slope of the shoulders just behind the horse's scapula. Then the saddle tree must twist to fit the slope in the middle of the back. Then the tree must twist further to meet the slope at the rump. And the saddle tree must accommodate the dip (called rock) in the horse's back which varies with different horses.
Hopefully, research in these areas will result in you knowing considerably more than I knew when obtaining my first horse.