A 2 tonne payload would be enough for two horses, unless they are really big. My cob's weight is 540kg. My 3.5 tonne horse box has a payload of 1170kg.
I would say 2 tonnes minimum, even if you're only going to use it for one horse (you've got to be able to sell it!). Once you start adding up the weight of horses, kit, water, fuel, people, hay, etc., you realise how much you need.