I know you all must be sick of this type of post by now, but I could really use the insight of your experience.
Anyway, I am considering getting an Arabian as my first horse after leasing for around 3 years, but I'm wondering if I'm wanting to do too many disciplines with just the one horse?
I would like to work the horse in reining, dressage, endurance, english equitation, competitive trail riding, barrel riding, pole bending, and possibly some jumpers. These disciplines are so ranged out, so I'm concerned that I'm asking too much of the horse. Or can it do both english and western styles of riding?
In other words, you want an "all around" horse, which can mean that it isn't GREAT at all of these, but rather maybe one or two and capable of the others depending on your riding ability and dedication. An Arab has just as good of a chance as any to do all of these during its lifetime. Rather than being particular about breed, try to find a horse that has a little experience in the ones most important to you. However, I hope you're not trying to do all at once. You'll need to get really comfortable and consistent in one style before trying another one so that you don't mix up your signals. However, once you're clear about what you're doing, the horse can learn to switch to different disciplines by feeding off of you and even just putting a different style of bit in its mouth for western vs. English.
The main troubles you'll have are between the extremes of disciplines that are very specific - namely the reining in western and the dressage/jumping/equitation in English. You really need to figure one out before switching to the other. Plus, each of those requires different tack, so we're talking some serious $$ if you're going to do both. As for games - as long as your horse is responsive, you should be good to go with either riding style.
Each discipline takes time and dedication. I'm an endurance rider and ANY horse can do at least the Limited Distance rides as long as it's healthy and fit, but most of the people in endurance are retired because they're the ones with the money and time that this sport takes. Any of these disciplines will require time and dedication, but endurance alone requires riding 20-30 miles per week to condition your horse and keep him endurance-fit, plus supplements, special gear if you really want to be comfortable and give yourself and your horse the best shot, upwards of $200-300 per event, etc.
If you're planning on giving all these a shot throughout the lifetime of a horse, then you should be fine. But focus on one at a time, or ones that compliment each other, so you don't get your (or your horse's) lines crossed.