Just like a good paint job; prep is very important. I am building an arena too and have been researching and talking with arena companies. Below is what I have come up with that I hope to be starting grading for very soon.
On a leveled area natural soil with at 1% slope, french drains on the parameter:
Base - crushed asphalt road base - Class 2 base… 3 inches. On top of that is 3 inches of DG (decomposed granite) compacted.
Then the footing - I would like to have washed concrete sand with Fibar (wood chips) helps retain the water and gives a nice cushion. Sprinkler system because I don’t have the room for a water truck 
There are companies that do put drains under the footing but that is much more than I can spend. It is already very expensive. But if we are talking a dream arena - I would also like it covered too!
My arena will be also be 100 x 200 because that is as wide as my property accommodates in the area I want to put the arena. I have ridden in different sizes and find the 100 x 200 is nice. A better size is 150 x 200 but 100 is fine if you don’t have the space.