hi i have two long necks and great to ahve but i must warn you that it may cost alot.
i started off witha four foot tank with one turtle my first one who was about 13 cm i was then advised on another forum that it was cruel to keep a turtle in these conditions becuase they will grow out of the tank(some peole may argue about this)i soon got a female and realised that when the time came when they wanted to breed placing sand on the dock for them to breed isnt good and she will porblay die from stress because she couldnt find a place to lay her egss.
so i built a pond with pond liner this was actually cheaper then getting a four foot tank and the turtles seemed more happy to be in their own habbitat.
so to answer your questions yes you do probaly need a pond with the right subtrate for land to breed them, yes it is ideally for them to hibernate to breed becuase then they know their going into the next season after hibernation which is the breeding season, i feed mine very second day, the males grow to around 26-28 cm and females i think are bigger so around 28-30 cm.
to actually keep this turtles in tank ,once adult size, its not so much length but width that makes them ahrd to keep in a aquarium. most aquariums are 35-40cm in width so if you imagine a turtle 30cm size trying to swim in a tank like that it would be very cramp, the pond i got with filter and liner and you dug a hole your self costs around 200 then add some plants land and water wich would be another 50 so 250 the turtles them selves seem to be changing prices i got mine for hundred each but ive see them for 60 or 70. tank would cost you around 400 which includes filter,heater,tank+stand and lighting. food which is frozen fish or frozen turtle dinners but mine prefer fish(very fussy turtles) will cost around 11 dollars each and lasts for at least four months. for a filter will for my pond i was told to create one just by buying a pump connecting to a hose which was connected to a canister which was filled with media whichw as then connected to another hose which let out water worked very well for me but with a tank i had a small internal and i had to do seventy percent water changes which were very hard each week.