Ditto to what loosie said.
Feed/nutrition and feet are what I'd consider most important. While a horse can get by on relatively poor nutritional balance (if not there wouldn't be many people with horses
) they'll do a lot better with it. (and don't believe what every feed maker says.....research the information....several colleges/universities provide a wealth of information)
You'll find that the most common and often life ending items for horses have to do with their digestion and their feet (and the feed can effect the feet). Many things that people still believe today is centuries old erroneous information. Many things that people are finding out (and often think are "new") are also centuries old, but didn't come out of Europe
so it wasn't seen as being what's best. Much of what is believed is the result of keeping horses in cities (for a long time there were huge numbers of horses maintained in cities for transporting goods and people around town). People were less concerned about the well being of the animal and more concerned about the labor they provide (also they were relatively cheap and easy to replace), but what they believed then is still carried on today.
While people in general do want to treat their horses better a great many still tend to rely on what was continuously passed down from those times.
Many good, well meaning people will tell you things that they believe, because it's what they were told, but not because it's right. E.g. Our vet during my youth told us for years that we needed to shoe our horses and especially my white mare with her white feet (there's an old belief that a black hoof is harder and a white hoof is soft..... it's not true
). It took 6 years before he acknowledge that my extended family, with our unshod horses
, had the best feet in the county (and was shocked when he found out that they were ridden on the hwy almost daily).
Proper management is critical, because without a healthy, sound horse the rest won't really matter.
The next thing after management will be the equipment that you use. Mainly the saddle. Make sure you get one that fits properly (and ideally displaces the most weight if you're not restricted to a certain type of saddle). A sore back can ruin your day and sure isn't much fun for your horse.