In terms of a rental car company I've used Omega on a couple of occassions and they have been fine but I think they are all much of a muchness. I think most companies recommend in terms of rental that you get one car to Wellington and then pick up another in Picton (with the same company) - because the ferry is expensive with a car. Although some high country roads are sometimes closed by snow, hardly anybody in New Zealand routinely carries snow chains. The only exception might be when you go skiing depending on which skified you go to.

I'll give some thought to some of your other questions over the next couple of days