Dressage doesn't have any action as fast as in show jumping; there's only walk, trot and canter, so the shutter speeds don't have to be really fast. 1/320 is plenty.
Dressage rings are 20m x 60m and few indoor rinding rings are that big. The indoor arena where my wife boards her horse is 60ft x 120ft, which is typical. Making an indoor Dressage ring is very expensive (making an open indoor expanse 60ft wide is expensive; making it 70ft wide means you can't use wood frame so it gets a lot more expensive), and few riding stables have one. Some riding rings have seating on one side, but usually it's on an end.
Lighting is poor, and uneven.I need a lens with a large aperture and a camera that can handle high ISO settings. It sounds like the K7 with the 77/1.8 ltd might do the trick.
BTW, wherever you are, you can find a local Dressage association at the USDF website: http://www.usdf.org/clubs/list.asp?TypePass=GMO
From there, you can find the website of a local organization, where you'll find the calander of events in your area. You'd be welcome anywhere, but clinics that charge admission for riders may also charge people who want to audit, so check first.