A couple of hundred liter tank will be good as a grow out, but they do grow large, so in a few years they will need a much larger tank, 1000L + (or if you are in the right part of the country you can put them in a pond instead.
You will need a heater yes.
Turtles can be stupid and and swallow stones, so it is best not to have any that fit in there mouths and they can block there intestines. So it is best to be big stones, or sand on the bottom.
You do need some way for them to get out of the water, if you buy a 'turtle tank' they come with a built in ramp that they can come out of the water on.
Turtles are messy animals for there size, so you will need strong filtration on it. I find a canister is best to put on it, they provide good filtration.
UV light is important, if they don't have one over there 'sunbathing area' then they can get shell rot.
Good luck with your turtle, they are fun animals and aren't that hard to keep. if you have any more questions remember to ask.