I know when I was looking to buy my easy entry cart, I hit a few different websites that listed the parts used to make the cart. Ctm manufacturing was a good site, they listed everything,and showed pictures. Like you, my husband was sure that he could build it cheaper, but once he priced out the steel, and expanded metal , and square tubing, he quickly changed his mind!!
-however, starting from the bottom up , you have 2 wheels-not bicycle, but heavier,so that you can drive in rough terrain without worrying about bending rims(motorcycle, or better) then an axle, springs a basket/floor,frame to put your seat on , springs, then seat, and seat back. In the front you have single tree, shafts, and there supports(usually adjustable) the shafts have a small loop on them to hook your breeching into.
There is also bracing that runs from one side of the cart to the other under the seat, and above the floor. The slow moving vehicle sign is attached to the back. The seat is bolted to the frame so you can adjust it to make everything level. You also need a whip holder of some sort, and a spares box is a nice touch, if you can find aspot to put it in.
I am sure I am forgetting parts, but hopefully someone else can fill in the missing parts.
Good luck and happy building.