There was a kid in my care home that had a couple their breed differentiated one was a yellow belly and the other was a grown red terrapin. Well anyway, the tank they bought which was fairly big was like 60 pounds, but it had the lights attached for the turtles to bask the turtles themselves were like 20 pounds together and the food was decently priced at 2 pound for like a lot.
I'm sorry I don't know the exact measurements, but it didn't replacing too often other stuff that was needed like a large rock was only around 20 pounds the turtles liked to climb on top that rock . Other things like clean water and a place to put the turtles in while changing the water can be usually just found in your house like use a small plastic box to put them in and regarding the fish thing. I would not suggest mixing the fish in the same tank as the turtles unless you want them to be eaten by the turtles which itself takes a few tedious days like when one of my carers suggested adding in three fish to be used as food it took like 2 weeks until their demise.