An easier way is to just add some sodium bicarbonate (NaHCO3), aka baking soda. Add 1/2 tsp for each 100 litres to raise it 1 dH. 3 KH is the absolute minimum that you need for a planted tank.
Yes the Seachem alkaline buffer was designed for raising KH and maintaining the ph in a certain range.
I would say 0 KH is the minimum for a planted tank. Many plant species like soft water.
I would not change the KH unless someone was having pH issues. There is nothing wrong with having 1 to 2 KH.
I perfer to have water that is as soft as possible without adding any chemicals.