Your base build up is OK, but... Two inches of sand is not enough, I agree with the previous post. You'll wear threw that very quickly and be running on the base underneath. Also you need to factor in wind erosion, every windy day, when the sand is dried out, you're going to lose sand, plus all the sand that going to get knocked out of the arena during use.
When we built our outdoor when have a good 6-8 inches thick of sand. You wouldn't need to go that thick. With the arena using for reining practice and the sudden fast stops. You'll need go go thicker then 2 inches. Our outdoor is slightly higher that the surround ground, just by a couple inches to help with drainage.