Actually for ADS competitions, there is no weight minimum. The 90kg is for FEI level competitions. I was checking the ADS Rule Book, page CD-22 shows the FEI requirements in black, with ADS requirements on page CD-23 in blue color. Very few CDEs in the USA are done to FEI rules, but read the information in the Omnibus or call the Organizer to ask questions.
90kg is quite light, only 198 pounds. I would be careful in inspecting welds, how the axles are attached to the vehicle. I would try to drive it before buying, but not always possible. You try to keep weight low in wheels and axles, it makes for a more stable ride, not allowing much in tippy feeling. But at only 200 pounds or so, a tall groom standing exerts a lot of force if they try swinging vehicle away from a post. This is where I said you NEED a bit of weight, but don’t want heavy weight because it just wears the animal out on the course. Add in type of going, mud, hills, sand, and that will add to his exertions pulling weight behind. Marathon at the levels above Training is supposed to stress him a bit with faster times, more gates in Hazards, because you believe Pony can do this, has the skills and conditioning to manage well. In most cases Driver and Pony have come thru Training to learn needed skills, going to step-up on competition levels. Nice to hear you plan to learn and not be strongly competitive while you gain the information, skills, learning needed, as beginners.
Here in Michigan we have a humidity problem to deal with most years. High temps and high humidity doesn’t allow good body cooling with sweat. Air horse breathes in is warm, wet. Something we have to figure in is how much we can ask of the horses in speed, going on the hills and rolling ground. We NEED to plan on aggressive cooling at Vet check and after Marathon most years. How fast can we drop temps and respiration in that 10 minutes we are allowed? How fast or slow do we ask them to go thru the Hazards? Other locations can let you compete in higher temps, low humidity with much less recovery issues. It does take good conditioning, he has to be able to self-cool as much as possible. Does make any dapples show a lot better too! Ours are all dappled, we have heard the dapples are part of helpful heat dispersal, show up on thermographic pictures.
We will be starting our conditioning soon, to get in at least 90 working days on the horses before the June CDE. Weather will interfere, along with that darn “need to work” problem so you have money to compete! Ha Ha Information we use says horse body needs 90 days of various kinds of work to get ALL the body parts equally conditioned, to prevent any single part getting damaged with too much work before getting it fit. This means 3 to 5 works a week, in the Interval Training system of getting conditioned program. If Pony is daughters riding animal, she can help do conditioning from the saddle some of the time. You just have to plan it, so she is doing what is needed to build him. It is not all about doing great distances or much at great speed. Speed and long distance waits until you are almost done with your 90 days, so none of his parts gets stressed too soon. Bone takes much longer to get fit than muscle.
That said I make certain that I condition the pony carefully and thoroughly for the job. Is he the fastest on marathon? Rarely, but that may be that I am not at that skill level yet. He is definitely very competitive in dressage and cones. I have never felt him suggest that he is being overfaced or struggle with the workload.
You can see the 10cm wheel difference in some of the pics - I found it did make a difference on size of animal to carriage - looks wise.