There have been a number of cart threads that have addressed your questions, check the "archives". But to answer your harness concerns, I would look for a carriage or CDE harness, and not a "show" harness if you are interested in comfort. Show harnesses are designed to show off the horse with a minimal amount of leather for a "brief" period of time. Carriage harnesses are more substantial and distribute the weight of the cart and passengers over a greater surface area. I also look for a steel tree in the saddle, with ample padding down the sides and gullet clearance, so there is nothing touching the horse's spine, especially if the horse is older. A good harness will outlast a horse, so plan accordingly. Breeching is required in my book, so the horse is slowing and stopping the vehicle with his big, meaty haunches and not his boney withers, which is what happens with just the use of thimbles or wrap straps. Again, these comments are in regards to the comfort of the horse, not the style trends in the show ring.
The one thing I will say about carts is that the less "springing" system there is, the more the motion of the cart/ground is transferred to the horse and passengers. Small coil springs are inadequate in my opinion.
"Buy quality, cry once."
