The essentials are enough space and feed stations to prevent bullying, good shelter and (ideally) varied surfaces to stimulate hooves.
If you are laying a track for winter use, you will need good drainage and a surface which doesn’t poach, but if you only need to use your track in summer then you can get away with just fencing round the edge of a field.
The trick is to be inventive - waste ground, old yards and areas of hard-standing are all great ways of providing varied, well-drained surfaces - and they come cheaper than “pony paddocks”!