Rural Heritage magazine had at least 2 articles in the past year or so where people did cost comparisons of farming with horses vs. tractors. Within the last couple of months, either them or Draft Horse Journal had an article on how many miles were walked/amount of time it took to farm a field of a certain size figuring the width of your average plow and other implements.
The breed/type of horse is dependent on what type of ground you are using, and your personal preference. A lot of smaller hobby/subsistence farms are now using Fjords/Haflingers/draft crosses because they weigh less, eat less, and over time cost less than draft breeds. You lose some pulling power, but that is manageable for many unless your ground is very deep and clay-like, in which case a heavy draft may be your only option. Most farms using horses for work have at least 3-4 animals, sometimes as many as a dozen or more depending on the ground, crops, and how many people will be in the field at once.
Minimum equipment needed for most farms is a plow (most use a riding plow now), harrow (probably two types), planter, cultivator, and a variety of wagons. Some older equipment would plow, plant, and harrow at the same time. It all depends on what your equipment is and what you need in your soil. For hay, you need a seeder, swather/mower, hayrake, and baler unless you're doing the old method of haystacks, which is fast becoming a lost art. Quite a few small farms that use horses use them for some things-- for instance, planting crops and cutting and tedding hay, then use a tractor for jobs like plowing and baling.
Draft-type horses often do light work at 2, but most hold off heavy pulling until 4 or later. If you're buying a team, older is usually better unless you know what you're doing. A well-cared-for draft horse can work well into his 20's, but again, that's variable depending on the horse, his conformation, and workload. A team that can't plow all day on an Amish farm may still do just fine on a hobby farm where they plow for 2 hours when the farmer gets home from an office job.
Good, broke draft teams for field work (not the fancy hitch horses) at a recent sale near here sold for between $4500 and $15,000 depending on breed, whether they were registered or not, how well broke they were, and color. Were I farming with horses, I would want at least 3 animals, probably 4 (so I could have two complete teams-- most teams will need to be changed out midway through a day in hot weather).
Our family farm depended on horses up until the mid 1940's. Tractors were purchased, but quickly sold early in the Depression, and the horses were brought back to work, which ended up saving the farm. With no payments on the farm or on equipment, it still produced enough to keep my grandparents and their animals thriving. They grew corn, wheat, flax, and hay, and had pigs, a few milk cattle, feeder cattle, and chickens. During it's heydey about 1915, there are photos of 2-3 dozen draft horses up by the barn, mostly broodmares who also did their fair share of field work. My mom remembers having teams of horses and at least one team of mules around while she was growing up in the late 1940's and 1950's. There was always one mule team in among the heavy Percherons because they held up to heat better, and one team could work all day in a hayfield in the heat and humidity of late summer when the draft horses struggled.