one of my turtle tank has that exact spec, 5 X 2 X 2 foot.
it has a 40 X 50cm sand pit so the water part is only actually 1125 X 500 X600mm. i run CF1200 and only clean it once ever 4-6months with a 10% water change and top up every week.
tank cost $300 but thats Auckland prices so expect to pay around $500