Well, the turnouts for dressage and cones must be the same so you can sneak by with two identical sets that way. On marathon, as far as I'm aware I'm the only one in the country who uses a Hyperbike! :new_shocked: LOL. So if you are seeking to show a typical scene, it might be easier to model a Frontier cart or the same one you're already using for dressage. Most people use show carts with wooden wheels for dressage/cones and a Frontier for marathon, or a Runabout type for everything, or a metal CDE-style cart with steel wheels for everything.
What makes marathon so different is how the horse is turned out. Colored boots on the big guys with colored pads and a matching colored halter left on under the bridle, the big war wagons have colored wheels and big wedge seats, etc. The minis don't get to have quite as much fun due to lack of available equipment and having a smaller "palette" that's easy to overwhelm, but you'll still often see colored pads and the tail done up out of the way with colored vetwrap. People at Happ's use vetwrap or colored electrical tape to temporarily decorate their wheels and whip, sometimes even the shafts. Mini sport boots have been impossible to find until this year but colored bell boots are common, especially on pairs, and some people decorate their browbands to match as well. Marathon is the one day where function takes total precedence over form. It's whatever works for cart, harness, and how the horse is turned out! For instance mud knots and regular straight vertical braids in the mane are perfectly acceptable to help with dissipating heat. The driver wears whatever is comfortable and coordinates with his chosen colors.
Hope this helps.
Leia
Edited to add: I didn't see Mary Lou's post- those are great pictures! I know someone was asking if I knew of any used ones, I should find out who that was.