Rear end

I was looking for this on google but the progress sucks, so I would like to ask you about this. How much hw can s13 axels and driveshaft can hanle, and does it need replacement for better/ lighter one.

Stock bits should be good for any amount of power you are going to make.

Unless you're planning some monster engine swap or something. But if you are then so much is going to have to be changed anyway that you can get to the rear drivetrain pieces in due time.