Equipping the Northmen mercs is already covered, though they have basic kit like leather or padded armour, and axes or swords (mostly scrounged). If you wanted to outfit them with better gear, you'd need to spend coin for that. That would, in effect, increase the pay but also the upkeep. In effect, you would have a chance for higher income at the end of the year if you made that investment. Right now, they are effectively militia. But if you kitted them out with better armour and weapons, they'd qualify as footmen, and earn more. Expenses would be higher as well, but you would have a higher range of possible income after the campaign season is over.
For all of you, as things have been fairly normal on your lands, supplies are abstracted. Your troops come to the field with the supplies required, produced by the manor. Depending on game circumstances, that may change in the future, but for now your lands produce enough that you are able to provide for troops in the field. For the Northmen, supplies come out of the wages you earn while being in the field, and get netted out at the end. Your people turn over any excess to you, after expenses and upkeep are paid. Down the road, if you are raided, or taxed heavily, or some other tribute is paid that you have to contribute to, you may not be able to provide your own supplies, so you would need to buy them. For now, it's all handled by your stewards, off-screen.
Having a spear-carrier is not considered a cost, but if you want them to act and serve as a squire, they need the gear and commensurate upkeep to have them serve that roll. If you want to upgrade a couple of Northmen mercenaries to act as squires, you'll need to kit them out (weapons, horses, armour) and pay their upkeep (£1 annually).
