I would absolutely have a conversation (phone/zoom/visit) with the trainer before sending your minis over and ask questions. It may be difficult for the trainer to recommend a cart before seeing and working with your horses and you, but probably could point out a few possibilities.
Part will depend on your goals as well as your minis, do you want to drive for pleasure on trails, show, both?
My minis went to a wonderful trainer that I’d both met and watched at shows over a period of two years. I wanted to drive for pleasure and also let my young granddaughters drive at mini shows. Knowing this, he kept them for extra time to ensure minis and children could both be safe and comfortable.
My first and only cart was/is an Easy Entry cart built by Triple C. It is well balanced with shocks and brakes and has held up through thick and thin. It was a great choice for us at the time.