Yes, they need UVB. UVB is important for all pets, but most especially for reptiles. (most pets will show improved health if you use UVB. But reptiles will get dangerously ill if they don`t have enough exposure to UVB).
As for tank size... bigger is better. Get the biggest tank you can afford. Don`t worry about it being `too big.` Remember, in the wild they live in rivers and lakes. Make sure the water is at least twice as deep as the turtle is wide. This is so they don`t accidentally get trapped upside down and drown. "Minimum dimensions for tank are: width - 3 times the shell length and length - 6 times the shell length." They need room to swim and move. But don`t go by the minimum. Remember, they will be growing. Eventually they`ll be 10 inches long, and you`ll need a tank that`s at least 30 inches by 60 inches. Your turtles should be about 5 inches long when they`re 5 years old, so maybe plan with that in mind.
They need somewhere where they can climb completely out of the water and get dry. Keep a heating lamp above the basking spot to keep it nice and dry. "The water should be 70-75 F and can be maintained with a submersible aquarium heater. The basking area should be 84-88F during the daylight period." I`ve heard some say that you don`t need to heat the basking area in the night. You can use any cheap bulb to heat the basking area as long as the temps are right, and you use UVB to light the tank.
Other things to keep in mind: filters. Get the strongest filter you can afford. Turtles make a mess. Cage set up. Try to balance what`s easy to maintain, and what`s fun for your turtles. Also: don`t put anything in there small enough for them to swallow (unless it`s actually edible like non-poisonous plants or feeder fish). Small gravel is dangerous. Keep stones about the size of your turtle`s head or bigger.
Try to feed a varied diet of veggies, commercial foods, and live food. And make sure they eat enough calcium.
Good luck! Reptiles are challenging pets.
~Rachael