What kind of horses are good? That's an interesting issue. First off, how far would you want to make it? You can climb to open easiest with an Arab, due to the built in endurance factor and intelligence. However, our highest ranks are also full of TWH, SSH, QHs, Appys, Paints, mules... Anything goes. The only things I would stay away from our heavy built horses (drafts and the like) and really big butted QHs, because they tend to get sore on mountain rides. Sky is the limit.
Expensive? Very. The rides I go to are usually over 300 miles away. Just fuel is expensive. You have to bring everything you need to survive a weekend -- most of your food, sometimes water... Entry fees are usually $80-100 for non-members.