That is actually the same sponge filter I use but the smaller model they sell, I use the fishmate 9000 available here without the pump:
http://www.fosterandsmithaquatics.com/product/prod_display.cfm?pcatid=10394
They are great filters except for 1 problem, they leak. I tube of silicone however can fix that problem pretty easy. On one I had a bad o-ring that I had to silicone to fix. Both of the had leaks coming from behind where you connect your PVC or rubber tubes, if you just remove the 4 screws holding those plates in and silicone it real good you won't have any problems. Been running one of them for about 6 months after fixing it and not one more leak, other one been good for about 2 months now. They do an excellent job of keeping your water clear, I would definitely go with the 9000 if that is going to be your only filter. I always run additional biological filters with these sponges, you might need to add a bit more bio capacity when the rays are full grown.
I think the stock tank will be fine, just make sure and find out that bottom diameter, as long as its over 6' I would say go for it.
To my younger rays I feed blackworms and then nightcrawlers when they get around 5-6". You can feed nightcrawlers younger but I don't like chopping them up, so I wait till there old enough to eat them whole. As they get old I transition them to tilapia, shimp, and silversides with nightcrawlers as desert.
I have forsale 2 5-6" motoros, 7 motoro pups, and 2 marble motoro x motoro pups.
In my collection I have leos, henleis, pearls, hystrix, castexis, motoros, marbles, mantillas, and a few hybrids.