Horse training is so easy that anyone can do it. Anytime you interact with a horse, you are training it, whether you know it or not, whether you mean to or not. You are either teaching them a good lesson or a bad lesson. Teaching a good lesson is easy if you know what you are doing. Teaching a bad lesson is even easier, especially if you don't know what you are doing. Horses learn, unlearn, and relearn things all the time.
You say that you wish to turn these horses into trail horses. I assume you mean for casual trail riding, not strenuous, hardcore endurance trail riding? Casual trail riding is all about the horse's mind. Physically, almost any horse can do it. Mentally, it requires a special kind of horse - a good-minded horse. There are some fabulous, very highly trained arena horses that simply lose. their. minds. on the trail. They spook and snort at everything and jig the whole ride. There are some crazy arena horses that I would trust my life with on the trail. It depends on the rider. Sure, an experienced rider probably can ride almost horse on the trail, but I want the trails to be safe and fun - for both horse and rider. Constantly worrying about a horse that bolts, bucks, kicks, rears, or spooks is a no-no for me.
What kind of experience do you have on working with horses?
Owning hobby horses is different than retraining horses, especially if those horses have already learned bad lessons.
Since these horses have been pasture puffs for nearly a decade, and you don't know their exact training, you do need to retrain them. Do not assume or guess what they do or do not know or try to pick up where they left off. That sets them up to fail. With horses, you get out what you put in. Retraining three green-broke pasture puffs requires a lot of time. If you don't know what you are doing, it can feel overwhelming, like you are being stretched thin. Horses, especially inexperienced ones, need consistency. Depending on your experience level, although you may not want to hear it, you may need to sell a horse or two. Horses do best what they do most. A smart, pushy horse can turn into a dangerous monster if not properly handled.
Without getting too specific, I'd just start with spending some good, quality time with them, doing the basic groundwork, and maybe some general desensitization.
If you are unsure, I suggest getting an experienced horse-person to help you in person. It is difficult to offer and execute advice via the internet.
"How much can a horse carry?" is a controversial question; therefore, it has been long-debated. "Can" and "should" are different, too. Some of those in the military carry fifty-plus pounds (twenty-three kilograms). They can, but studies have shown that that is contributing to arthritis over the course of several years. For horses, the general rule of thumb is no more than twenty-five percent of the horse's body weight. This includes everything - the rider, the tack, and whatever else the horse will be carrying. I believe that most people go by the twenty-percent rule, though.
There are a lot of other things to take into consideration besides percentages. These things depend on both the horse and the rider. From the horse's perspective, things such as fitness, hoof health, bone density, and conformation (such as back length) affect the horse's carrying capacity. The horse's height does not directly determine the horse's carrying capacity. A lean 16.2 HH Thoroughbred will probably have to carry less than a strong 14.3 HH stock horse. From the rider's perspective, skill plays an important role. A rider that is self-balanced and with the horse will do better than one that is unbalanced and a dead weight.
For a horse that is stiff, it is best to first consider the degree of stiffness and why they are stiff. While some stiffness is minor, other stiffness is more serious, like joint problems. Joint problems should also be taken into consideration. A horse with arthritis should carry less than the exact same horse without arthritis.
P.S. Horses are measured at the withers in hands. A hand is four inches (ten centimeters). If the horse's height is not of four, the remaining inches are put after a point. A horse cannot be 14.7 HH. The max is 14.3 before turning into 15, 15.1, 15.2, and 15.3.
Do not leave halters on horses out in pasture unattended, especially if those halters are not a break-away.