On 2000 acres worms are much less likely to be a problem than they are for pet horses and ponies kept on the same small patch of ground all the time. Also the other species kept will help to break the cycle as they ingest the worms but the horse worms cant survive inside them (and vice versa).
It is something to be aware of though just because they arent likely to have been wormed; a friend did have a welsh pony die from horrendous worm damage who was bought from the breeder off the hill, but in that case they werent really on the hill, and there was much greater overstocking. The New Forest people found that a once yearly worming at round up was the best balanced approach for the wild ponies there.