Q3: piston ring material depends on block material (bore surface metal). P/ring material & intented use (stock, race, turbo, nitrous etc.) determines ring gaps. Block & piston material and intented use determines bore to p/wall clearance.
So near stock is where you want to be. I think.
Before you buy some fancy nos pistons super cheap check that there is rings available. Modern rings are thinner and 4 mm oil rings are not nessesarily on every manufactures production any more.