As an Italian would say, la vedo dura. It would be tough.

First problem: "Low Class". In the middle ages, the "Warrior Class" was synonymous with "Landowning Class" (you need money to pay for arms, armor, and a horse. Even infantry in the form of sergeants or urban militias were professional soldiers in the former case, and freemen in the latter case) and little changed in the early modern era.

The first professional soldiers in Italy not tied to the traditional Lord-Vassal arrangement appeared in the thirteenth century after the Crusades, when Germano-Frankish soldiers returning from the Holy Land were left stranded in Italy. Often little more than roving bandits, they were often rounded up and put to good use during dynastic infighting.

Subsequent mercenary companies were more often than not veterans of fighting in France and/or Germany, where dynastic fighting had blurred the lines who, precisely, was fighting against who, as individual vassals and the forces they commanded opportunistically backed different sides based on plunder, risk/reward, and who seemed to be winning. The first "famous" mercenary companies came from places peripheral to the larger conflicts of the time (and were often veterans of other, even earlier, conflicts) and were composed of men with little to loose and everything to gain: Catalans, Basques, and Bretons preceded the English, Welsh, and Burgundians. Most early companies were mounted men-at-arms; men who had been born into landowning families, but perhaps stood to inherit little more than a horse and a name. These men were quick to attach themselves to powerful landed lords, offering their service in battle in exchange for lodgings and a place in the court. Following their lords to war away from home, many stayed behind to plunder and offer their services to whomever could offer them the most.

Many of these companies had slowly trickled into Italy in the 14th century, being employed first by the Marquis of Monferrat, and later by the Visconti in Milan (many anecdotes could be told here. My favorite involves Lodriso's betrayal of his cousin Matteo, but really, I'd never finish if I started).

When the Estates General of France agreed to fund a single professional Royal Army in 1439 these companies were given the option to either join the new army or be hunted down. Many entered the French Army, contributing to the eventual resounding French victory at Castillon, but others picked the high road, trying their fortune in Italy.

The later appearance of Italian mercenaries can be attributed to a sort of snowball effect. At the late date of 1377, Alberic of Barbiano established the first Italian-only mercenary company. Alberic himself was a son of the Count of Cuneo, a moderately important fief of the Marquis of Monferrat, and was most probably present when his father was called to arms in the conflict between Monferrat and Savoy. Instead of returning home at war's end, Alberic, then sixteen, joined John Hackwood's White Company, a company of English veterans of the hundred years war employed by the Marquis of Monferrat. As the company traveled through Italy, many young men similar to Alberic would join.

The White Company would go on to be employed by the Republic of Siena, and later, the Papacy. When the Pope, in the pocket of the King of France, was convinced to move to Avignon, Hackwood found himself fending off a land-grab as not only cities under Papal control tried to assert their independence, but also incursions by Florence, Milan, and Padua. After witnessing Hackwood's forces raze the rebellious city of Cesena to the ground, Alberic left the company. Disgusted by the unchecked antics of foreign mercenaries, as well as with the Papacy which at this point was already a pawn of foreign powers, Alberic pledged his services to Barbanò Visconti of Milan, rounding up the Italian mercenaries who would follow him. Alberic's Company of St. George, as they came to be called, would serve as an incubator for many mercenary captains of the next century.

Legend has it that during these wars in central Italy, a young man called Giacomo Attendolo was ploughing his father's field when a group of knights in the service of Alberic of Barbiano rode past. Dismissing the absurdity of a landowner's son ploughing a field (although Italy was recovering from the plague. Shortages of manpower makes it more plausible than you'd think), the legend goes that young Giacomo, bored with country life, decided to throw his plough into a tree. If it fell back to the ground, he would get back to work. If it stayed stuck in the branches, he's join the company. The plough stuck. Giacomo Attendolo rode after the knights with his horse and his arms.

The story might be questionable, but Giacomo Attendolo did become one of the most celebrated mercenary captains of the early 15th century, first under Alberic of Brabiano and later with his own company, first in the employ of Perugia, and later becoming Cheif Constable of the King of Naples. In his career, he would earn the nickname Sforza, or "Strength", and his son, Francesco Sforza, would not only become the greatest general employed by Milan, but would also marry the Duke's daughter and only child, inheriting the throne of his father's first employer.

Legend aside, the Attendolo-Sforza family's rise is attributable to one thing: Giacomo had a horse, arms, armor, and had enough free time to have learned out to use them. It is conceivable that his father, a large landowner, had sided with the Milanese army (either to avoid pillage, or he really did hope in an independent Romagna), and Giacomo just stuck around for the ride, the same way Alberic had stuck around Hackwood's army after the war against Savoy. They were young men who had enough means to access these opportunities, and little enough to lose that they seized them when they came.