I like PTO mixers. You throw in the water, cement, gravel and sand where it is on pile and start driving to where you need the concrete. Before you get there it is mixed. It all depends on what you are doing. In example I can't imagine that you would carry sand, gravel and cement as you progress from post to post. But I can imagine that you drill the holes first and then you mix concrete where the sand and gravel is and take it to the fence line.
I have a PTO mixer with hydraulic lift (Top link). It damps by its own weight. Bought it new for $1300 to give you an idea what new costs.