Mine attaches to the hub, via hitch that attaches to the hub axle skewer. Simple to set up and quick to hitch and unhitch. Never tried a seat post attachment, but would imagine the hub attachment is more stable. Seat post might be more maneuverable in very tight places.
I think they tend to come with 16" or 20" wheels, perhaps look for the bigger size for rougher roads.
Also I'd look for one without, or at least removable side frames. If everything is boxed, bagged or lashed securely they do nothing except get in the way and add weight.
I think you can get a good one off eBay for about £60 to £100. Mine was originally a pet trailer with the canvas compartment bit removed to make it into a flatbed.
I've covered well over 1000miles with it, all sorts of loads, camping gear, shopping, firewood, my elderly dog, children, other bikes, inflatable canoes, kitesurf gear etc etc.