Stingray Tank Setup Question

Congrats on deciding to get stingrays, you'll love them!
First, you are going to want to stick to small stingrays in a 100g. I recommend Hystrix, as they stay at about 12" disk size.
With the sand, make sure that it will not irritate your stingray. I tried playsand and it was irritating to my Hystrix. Some people have had luck with this cheap substrate. If your ray's belly remains pink for a week or so after you get them or your ray never sits on the bottom, you'll have to change out for a "softer" substrate. I am currently using Estes brand sand and my ray loves it. You don't need much, I only have about 1/2". In fact, many ray owners keep their tanks bare for ease of cleanup. The rays don't mind either way.
With the filtration, I would say that you can't have too much. Rays are especially sensitive to water quality. The best way to keep your ray happy is through many water changes. For the first month that I had my ray, I changed 50% twice a week. Now I change 50% once a week. Rays eat meaty foods and therefore produce a lot of waste. Frequent vacuuming of the substrate is essential along with frequent water changes. As for your sump, this is a great way to filter your tank. Putting the heater in it is a wonderful idea, because rays have been known to burn themselves on heaters. I am not a sump expert, so I cannot say if it is adequate. You would have to tell someone more experienced than me how many gallons you flow per hour, the volume of bio balls, and your water parameters. I suppose that the best way to know is just to buy a good test kit and monitor ammonia and nitrate to determine if your sump is enough.
Once again, for water movement, you would have to make that call. My tank is fairly calm, and I have no problems with my rays. I would recommend trying different things out and observing your ray's behavior.
Good luck and let me know if you have any more questions!!