First of all, make sure he can do "mandrel" bends, vs "crimp" bends. Crimp bends reduce the inner pipe diameter, and don't flow as good as mandrel bent pipes.
Also, make sure he can get nice and thick 3" metric flanges, there is no SAE exhaust flange that exactly matches upgraded exhaust flanges on our cars.
On top of labot/material cost, You will still need a muffler, something that looks and sounds nice.
I say go buy APEXi N1 exhaust for $400 and forget about it.