My suggestions are oriented towards comfort in the vehicle. The very light 2-wheel carts seldom have much on springing under the seat to make for a good ride on uneven ground. Many are designed for use in rings which are groomed. They tend to be a basic frame with wheels, may or may not have a backrest, which I consider essential on drives over 20 minutes. Seems to be especially true with mini equipment, though better designs are available at higher prices. Two wheelers give a rougher ride than 4-wheelers because the shaft go up and down with the animal being one point of the balance. 4-wheelers hinge at the shaft,/vehicle connection, so any horse up and down motion stops at the hinge point. Never affects the people in the vehicle. Really nice if you have a big moving equine.
I will second getting lessons with a driving trainer, learning if he thinks it will be fun. Not sure of your location, but there are trainers out there to learn from.
As to a driving animal, you often “get what you pay for” in their training, being fun to use. Shopping among the Amish, horses tend to only have one trot, no bend to speak of. They may be fearless on the road, but lack barn manners and other things riding folks consider essential. So they need training. Listen close to what seller says about horse, ask MANY questions to cover details. Amish kids make money training and selling ponies, minis, so many of them have extensive handling, road time, fearless in odd situations. Check that they stand well anyplace, the BEST gait on a driving horse!
Other smaller equines like pony size Morgans, POAs, Shetlands, Hackneys can be fun driving animals. May know driving as part of their basic training. But very small animals can only pull reduced loads, if he might like taking a friend or OP along for a long trail drive.
As mentioned, it is not a cheap hobby, but quality equipment stays nice with care. There is LOTS of used equipment at very good prices, to save on costs.
Driving folks seem to be the nicest around, very helpful. There ARE other men who drive, he would not be the only guy at a club gathering or fun drive.