How about this:
Sheriff apointed by mayor.
OR
The sherriff could be a NPC. They can be trained in the town hall. They would have normal skills/attributes. For a certain amount of money, people could use 4 hours to teach the sherriff something. The skill attribute they are teaching must be lower than their own. For example, a person trying to teach a sherriff 27 aim must have at least 28 aim themselves. They could also buy/give the sherriff a new weapon.
BOTH OPTIONS:
Sheriff gets triple tactics (to discourage other people coming). Anyone can attack. They choose a store before battle. If they beat sherriff, they raid the store and steal one random item from that store/bank. Could be a broken wine bottle, could be a quarter horse. If they raid the bank, they steal 1/4 the town treasury.