Horse actually take better to driving than riding. If she's been a nice sensible riding horse she'll be a good driving horse. The horse does need to get used to the shafts putting pressure on her body when she turns but if the first turns are big wide one's she'll likely be fine. Carts are often noisy altho I doubt that will bother her. For your first driving have a lead shank on her halter under her bridle and have the helper walk by the horse's shoulder, with some float in the lead line so as not to influence the horse but is there "in case". You don't want the horse to be able to see the person or she will think she is being led which can influence the horse's thinking. Good luck and have fun.
I am going to disagree with several statements above.
Horses already trained to drive may take to Driving or they may not. Depends on the individual animal. We have a 50-50 success rate with retraining older animals. The mind needs to be willing and cooperative in the learning process. Some horses are only EVER going to be Riding horses, don't have the mind for Driving. Not YOUR fault, they are what they are. We had one who HATED driving, and we were not going to change her mind!! Both her sisters drove VERY WELL! You can consult with a trainer about the horse, but they may give you the same answer, "Just not a Driving horse". Driving asks a great deal from a horse, not all are equal to the job, because it is harder to be a good driving horse than to be a good riding horse.
We start everyone working on the long lines with an open bridle, surcingle or harness saddle to keep the lines off the ground. You want the long lines to be LONG, to make BIG circles around you in the center. Our lines allow a 60ft circle, probably about 35ft long, to go around the horse rump for control of both ends of the horse. Lines run thru rings or shaft holder loops (if using a harness saddle) midway on horse barrel. You NEVER put lines thru rein rings along the spine of horse to work him. He can spin UNDER those high-up lines, get all tangled, pull loose to runaway. Lines along the midway of barrel keep him BETWEEN the lines, for much better control of the whole body, no spin-outs! Rein handler is MOVING inside horse in a smaller circle, NEVER standing still. This is a working and learning exercise, not just "getting the fidgets out" kind of thing that most lunging is.
Rein contact is given and taken, never static like with sidereins. You can reward his face instantly when he give that 2 seonds of vertical face, build on the magc moment! From the side you can tell if he is actually driving from behind or rearend is just tied on, face is giving to the bit or just hanging on the lines, so you can fix the problem. We want horse in open bridle to see what is going on behind and around them, NEVER afraid of noises or weird things that they eventually will pull. You can work in a blinker bridle now and again, help horse get used to it for later work.
The handler with lead at the horse head is good for starting the animal on long lines. Handler responds to the Driver's vocal cues, gets horse learning the words as they go along, work out the feel of long lines in the big circle.
Horse is expected to quickly respond correctly to the cues, so leader person can be dismissed. Name of horse, WALK ON, Trot, TROT ON, with voice emphasis, are usual cues. Whoa is also done with the Leader, stops DEAD, stands still until told to move again. This Whoa is to be ABSOLUTE stop, practice with the Leader until horse "gets it". Start with short sessions, couple seconds, then quickly ask horse to move so he doesn't make a mistake by moving himself. You can also practice stopping with just the halter and lead, making horse stand quietly, feet not moving on the way to pasture, down the barn aisle. Use your daily routine to reinforce his training for Whoa, with no extra time needed! Amazing what that 5minutes with a couple extra stop and stand will do for him in short order.
Have to say that having that Whoa/Stand-still on your horse may save your life at some point!! You can't ever allow horse to move off after you say Whoa, he has to turn into a statue for the time you ask. Yes, you start with whoa and VERY short stand, then TELL horse to Walk On. YOUR CHOICE to move, not his!! You build up during training time to longer standing times, being very quiet, praise him for it! NO SNACKS! Whoa and Stand is the BEST gait of any Driving horse, will allow you people to correct any bad things, fix problems, while he patiently waits for the next direction from you.
If you have never trained a Driving horse, you should check around for Driving Trainers locally to get some lessons from. You will be more familiar with how horse SHOULD respond after using a trained school horse, learn about harness fit, adjustments and fitting vehicles on the horse. They will also be helpful to you in the first times of hitching, spotting "issues" quickly with their experienced eyes.
Doris Ganton wrote a book, something like "Training the Driving Horse" which is an excellent tool with great photos. Often available on Ebay or used book sites. Not all training books are created equal, but I would recommend hers. Well worth paying for Driving lessons and having the book, to increase your knowledge and personal confidence.
I do want my horse to know someone is beside him if he needs that Leader person. We put the Leader person back beside horse when we do new things, add equipment, step up to dragging items. Person may be doing nothing except walking along holding the rope, but you are developing his confidence, showing horse what you want in this new setting. If you NEED Leader because horse ignores the long lines, is not paying attention to Driver with further steps like starting dragging a tire, feeling pressure of pull, then Leader person keeps horse going in the direction needed. This is NOT a training failure. Leader again is showing/teaches horse what you want done with NO CHANCE horse can do the WRONG thing and learn to escape. Having the Leader gives horse confidence to follow, go thru the steps and then finally do whatever, alone on the lines. Then horse is lavishly praised!!
Know going into training for Driving, that there is not a set time for horse to be ready to drive. No 30-60 training times. You have to go with what horse tells you he is comfortable with, COMPLETELY obedient at, before you can move to the next step. Backing up to review and improve a step is fine!! Horse must totally understand the step, respond correctly, before you can advance. Each step is built on those you did previously. Missing a step, not understanding the step, means he has no base to know what to do when things go wrong! Some stuff will go quick, other stuff will appear to take forever before horse "gets it". You MUST have ALL these good basics on horse from the ground, before EVER thinking of hitching him. He has to BELIEVE in your partnership, know that he can depend on you to NOT HURT him when you ask him to do these things. A lot of the ground work is "partner building" in gaining his confidence with you giving orders. This can't be hurried, takes time.
Along with the lines and surcingle/harness saddle, you will need a whip for your other training aid. I suggest getting one with a long stick, 5 1/2 to 6ft stick, and a long lash. Regular lunge whips are seldom balanced, have too short a lash, so they are hard to use for very long. We use inexpensive Driving whips, $20 or so, and add parachute cord for more lash length. You MUST be able to "reach out and touch him" wherever he is out on the lines. Just getting a flick, go FORWARD, makes him pay ATTENTION to what you say!! You are not beating him, but whip use is part of driving. He will get touched with the whip for turns, bending, so he should respect it but not fear it. Whip is your "legs" like if you were riding, touch means move, bend, step over, like closing your calf on his side from the saddle. Whip should be light, balance is usually higher up than the handle part, for holding without whip pulling hand sideways. Get used to carrying whip, using it, since you should plan to carry it all the time you are driving in the carriage. Whips are a tool, useless if stuck in the whip holder! If you need the whip for a dog attack, you need it in your hand RIGHT NOW!!
These folks are really nice, have the inexpensive whips and pretty much anything you need for driving. Good quality stuff. I get whips from them and add my lash length to suit what we need it for. Country Carriages USA
Good luck with finding a horse. You might check around for other animals before going to see this one, find one already broke to drive AND ride. There are LOTS of horses available going into winter, so don't jump to buy if you have any doubts in a particular animal.