The turtles will need 10 gallons of water PER inch length of turtle! The filter is needed, foggy or dirty water will cause sickness to the turtle, clean healthy turtles will not get salmonella or sick! The basking area must be 90 degrees, as the water temperature 75-85 average. UVB light is IMPORTANT for a growing/ANy turtle! ^o^ people will illegally sell baby turtles 4" and smaller (as Coursair said) so DON'T purchase any terrapin under 4 inches!!! You can be gifted them for free, but don't purchase.
The diet is very important for the turtle as well, cabbage and spinach are NOT to be fed to water turtles! But romaine lettuce and carrot/apple chunks are delicious!
As for the tank, if you like to have fish, beautiful pet fish sharing a tank with a turtle, bye bye fish! Turtles love to chase and eat those delicious colorful snacks. Feeder goldfish and little shrimp can be quite yummy! Reptomin is a very good brand of turtle food, you can also buy turtle treats! And if you're going to have a turtle with little kids, make sure they are gentle and wash their hands after handling it! Kids may want to paint the turtle shells for fun, but paint will stop absorption of UVB and UVA light for the turtle. Paint will also trap bacteria on the shell.
For a happy healthy turtle, it MAY be expensive! A $10 baby at the swap meet will end up a few hundred, if you want it to live long, healthy, and happy! ^o^