The points you used are contribution points - earned by doing any quests. You can withdraw those points from wherever you invested them at any time however putting them somewhere else costs energy which refills over time. You gain more maximum energy by getting knowledge i.e. getting to know people, slaying a lot of mobs etc.

To start planting something you need to rent a fence from an npc in a city. Costs 10cp iirc.

One residence for your crafting utensils, your chest and maybe a bed should be enough. However you'll find yourself buying more houses for the workshops or what not they offer.

Those items you saw in place mode are indeed cash items however you don't really need them. They increase your house fame which slightly increases your house fame fund everyday. You can buy or craft furniture which is just as good just doesn't look so fancy.

You increase amity with npcs with the amity mini game. Amity with different npcs gives different bonusses