Thanks. That is an easy entry cart. New they sell for about $500 to $600, wooden carts are more expensive and more up-keep. It is a good basic beginners cart. Harness start at $150-$200. They come in nylon and leather. Show quality is much more expensive. I start them with just the bridle and surcingle (girth). When they carry that well add the breast collar and breeching (rear end stuff). Just tie the straps together that would normally tie up to the cart. I teach them to turn and stop (as in don't move your feet at all) by the bit on the ground, and the all important word, whoa. They need to know the whoa command by the bit and verbal. Do be sure he is comfortable with the cart before attaching it. If he is afraid with it attached it is like the enemy is right on his heels and he will run faster to excape it.
The Other Mother, no reason older horses can't be trained for cart.