Thanks for the advice guys! This is southern oregon, not sandy soil unfortunately, it seems more of a loamy/clay mix. I have decided to just go with a sand arena, with about 3.5 inches. Rubber can be added if needed. They get about 12 inches of rain each year, fairly spread out.
For the barn building, does anyone know of any good two stall barn plans? With an attached feed and wash stall? The barn would also have to hold the hay, an atv, and tractor, plus some shop space, off to one side. Unfortunately, we won't be able to build a separate barn for the hay, but we will put up a couple fire sprinklers and build a "fire wall", separating the horses and hay/atv/tractor space.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown