I know this is manga, but medieval plate armor would cost the equivilent to a modern person buying a Bugatti.
What kind of village is this for simple watchmen to all have them?
An insanely corrupt one. I pointed this out in one discussion for this chapter somewhere (probably on reddit, since MD only recently got comments back), but Philia mentioned a few chapters back that all the girls "sacrificed" to protect the village were really just gangraped by the mayor and his cronies before being sold off into slavery.
Depending on how much young, female slaves go for in this world, that could potentially finance a platoon's worth of plate; plus, the mayor was mentioned to be an ex-soldier, so his might have been from then.