Aquarium? Unless you want to spend a lot of money (multiple grand), look on the marketplace here and craigslist. A new 180G (6x2x2) is upwards of 800/900 new and what you want is a lot bigger, and will cost a lot more. Eels are easy, rays I'll leave to the experts. But you'd want to consider the footprint as most rays need 3 ft wide tanks (or wider?).
Filters? With a tank that large, you're pretty much going to have to build a sump (unless you want a bunch of canisters). I'd buy a used tank of cl and make it into a sump. I'd probably get the pump of ebay.
Fish? We'll worry about the setup first!
Welcome to MFK, there's tons of knowledge to be found here. I recommend scouring the forum using the search forum. You'll have a lot to learn before you're ready to take something like this on!