Fortunately he has said he will pay whatever it costs so it should be OK. I will ask them to blend the colour onto the front and rear wings to ensure the colour looks right as suggested by Jimmy on here.That's at a very good body shop I know I can rely on. I can't take chances on a nearly new car.
With the Z3s I'd look for a decent back street shop. There's an excellent one in Swansea I'll give a go at some time. I'd hope you'd get your damage fixed for £500 or under. And that would include pulling the dent out or another door.
You could do some of the work yourself to keep the cost down. It's easy to strip the door. And easy to swap another one on if required. But it could save you £100+.