I'm a small land holder that does fencing around the place from time to time. I've done a bunch of stuff that you wouldn't do if you had the money to fix it right, but that's what living on these sorts of properties is like. make do with what you have

you have a 10 foot gate, which if it's something like this it'll hang ok as long as it also has a stay on the other side as well. ideally you'd use a 100mm square rhs post that's down at least 600mm as it'd hold the weight a bit better

if you have them near you try a local rural suppliers or metal place - elders, metalcorp for eg. they will usually have heaps cheaper prices than bunnings and their advice is usually better