Does RoC season 2 start from the previous S1 building levels or we have to start from each building..

The building are reset with each RoC event so you must build them again from level 1.

You have a short period of a few weeks between RoC seasons and all buildings will need to be replaced. I would recommend having your alliance begin to save up stones (I save 14k for my buildings) and donate what they don't set aside for their buildings, so you can quickly build all your Alliance Centers.