Yes, I found one last year that I love and my mini loves to pull - it was a bargain at $60. I had to cut the shafts, paint it and add a singletree (it only had hooks for the traces). However, it has no springs under the seat and although I loved the way it goes, it most definitely needs a spring. My back couldn't tolerate it. We are going to add a carriage spring sometime (when my brother has time) and put on cart wheels. (So, was that a bargain? - well, sorta, but it was fun.)
Now I have another one ($435) - it's an older cart that could use refinishing but has a great spring under the seat. It is a hair too narrow (shaft space and wheel base) but is great for my current use. It's wood with a square metal tube framing. Lacey likes it although it is heavier than the other. I think with refinishing it would be ok for showing. A narrower horse would fit better between the shafts, but she likes body contact, so it works fine.
Now I'll admit that I'm not an expert, and I'm a bit eccentric - oh well!
but I've owned two of those pipe EE carts of different brands and I have a core loathing for them. No more of them will come through my revolving gate!
However, many people recommend them as entry level carts. I do wonder why so many are available for $300 (well, truthfully I don't wonder at all, haha)
Best to stick to the names mentioned earlier - maybe you will have better luck with those.
For sizing, I am told that the shaft end should come to the point of the horse's shoulder (no further), and then between the horse's tail and the crossbar there should be 10-12" of space (or was that 8-10"?)
You don't want the horse too far from the cart (more weight on the saddle) yet you need room for their legs to move out behind, and you don't want the crossbar hitting them in the bum when the cart stops.
So if you measure from the horse's shoulder to the length of where the tail is (straight back, not curving around the butt), and add 10-12 inches, you would know what length shafts you need.
Then there is your harness, which will need the right length of traces to allow the above hitching set-up.
Best to get an expert to help you! It is easy to make mistakes through being too optimistic.
Wilma