Parish buildings can be demolished at any level and the parish will retain the bonus from whatever level they were demolished at. So the idea really is to build a building, max it out and then demolish it. The goal you should reach for is to build and max every single building and be left only with the buildings you are not supposed to demolish.
Having more people in your parish means buildings and gets upgraded much quicker. Remember if you demolish a building leave a post in the parish information section noting what you have demolished so future stewards can see what has been already placed. Donate as much as you can. Levelling up buildings benefits everyone, and with multiple players donating everyday it will not take long to level up buildings. The most important buildings to really place in the parish capital in my opinion to benefit the whole parish are the turret maker, the ballista maker and the Tunnelors Guild. Having all these three buildings will allow every village in the parish to make turrets, ballista and a tunnel in their city which in itself is a massive defence. Also the reason to place these early is that it takes a lot of weapons to upgrade them so start early.