I was seriously looking at running 18's when I ordered my wheels. There were some good deals going on the Michelin Pilot sports here too and the tyre bill for 18's wasn't much more than 17's. I settled on 17's because there is a better range of drag radial style tyres as I want to have a go at the strip from time to time. The selecton of 17" tyres in wide sizes is a lot better than 18" if I want to run a different brand/compound.
My wheels were built by an Australian company called Ozwheels (no relation to OZ racing). They are a 3 piece design using a cast alloy center bolted into 2 spun alloy rim halves. Ozwheel build a lot of wheels for racing Porsches and Ferraris down here and they make a very nice wheel.
I wanted to run larger diameter rims since it gives you a lot more room for bigger brakes. Also the offsets I'm using would look a bit odd for 15" wheels. It's an aquired taste but I really like the look now. Move over Ben Hurr, there's a new charriot in town
The 18" versions use the same center as mine and they just bolt them into larger diameter rim halves. Changing rim widths or diameters is easy as you just need the new rim halves spun up and then un bolt the centers and bolt the new halves on. Check that they run true and away you go.
Cheers