I think that it will be very very problematic to hire mercenaries or even something like soldiers by yourself. Such people are expensive and you are some kind of a young men with nearly no money and no noble title which would grant you anything besides a life of a poor blacksmith.
The lords and ladies had to pay tons of money for their armies and every month of war is killing your state treasury for sure. And if you run out of money nearly every solider will go crazy and fight for themselves and starts a carrier as a robber.
The only thing I could imagine would be that after some year of military training the player character would become something like a seargent who could command at least some soldiers at battle or in a younger state some pool people in a village who are defending themselves against robbers.
