Hi Rob, surprised I've only just seen this, welcome.
Our fence posts vary, boundary are 2m, internal 2.5, old ones are 3m apart.
I think posts are 5ft, x3". Your local agrisupplies would advise.
I would get it fenced asap, when we bought some adjoining land we realised a local farm was using our gateway as access, they actually had a skip put in our field, we had it fenced and locked gate before they could claim any rights. Then found they were playing with their quad in our field, they had actually put a gate from their yard to our field, we didn't touch the gate but put some posts and fence across on our side, didn't have any more problems.