Personally I wouldn't keep the grass, it gets worn down with too much use, muddy when it's wet out, and slippery with rain. I don't know where you're located, but that's the big problem we see here.
I would go for sand, with a layer of limestone beneath it. I think I heard 6 inches of sand is good, but don't quote me on it. Builder's sand is a cheap alternative, but if it'a business facility you'll want to invest in sand made for these purposes. I'm putting sand in my grass arena this spring, not sure what kind though. I have heard that Clopf Fibre is good, here's a link: http://www.mceltd.com/?page=which_surface&product=clopf