The biggest problem is clearing the spring mount in the rear. If you go with a 19" tall rim you have no problem getting up into the rear suspension without hitting anything. You actuall end up getting the rim around the fixed mount suspension stuff.
It looks like a 5" off set with a 10" rim will work up front (at least on my ride with the fender flares). I have seen plenty of sharks with the wheel adapters and late model 88-96 rims (which are 9.5").
Of all the things I wish I knew about C3's, right now I really wish I knew all the possibilities on wheel spacing. I am going through some of the same quandries. However, I am looking to jump to a 12" on the rear if at all possible.
The other trouble is, if you measure wrong and have a custom rim shipped to your door---you bought it. So, measure two, three, four times, and order once!
I am thinking about building a measuring device out of plywood that would allow me to simulate the rim, and move the lip in and out to determine exact fit.