It would be best to model this shape without the wheel arch cutout.

Then after creating the network surface, you can then use trim to cut out the wheel arch.

Strangely enough I had to pick the curves in a certain order for network to work. If you just select all the curves with a crossing window it will give a bunched up result.

You will find some car modelling tips here http://moi3d.com/forum/index.php?webtag=MOI&msg=6512.1

I do agree with Andrei though, polygons is the way to go for organic shapes like cars and aircraft. It can be done with nurbs but you really need a package with a very advanced toolset that would cost big $.