If you want it to last for a long time (ten seasons), then be careful not to run it too fast. Think of it as a horse drawn spreader, and go about that speed. Finding broken parts will (may) be tough, if a cast part breaks. In the winter, don't let manure freeze in it. Fill it with warm manure, spread it right away, and clean it up after spreading, before anything gets a chance to build up. Oil it up well, and keep the bearings greased. It looks in pretty good shape, without much rust (best as I can see by the pictures).
I remember as a boy, driving too fast (road gear) coming back from spreading a load. The cast cams (I think there were three lobes) used to advance the bed slats broke, and it wasn't worth much after that. As poor as we were, this didn't do my father any favors.