After further emails with Korst Banden, the local retailer of Compomotive wheels in the Netherlands, it looks like the 3 piece PC wheels are out of the running since they're about twice the price of the MO wheels.
I sort of set myself a limit for how much I'm prepared to spend on wheels for the Ghibli and that's about 10% of the value of the car. Unfortunately, the PC wheels are just too expensive given the value of the car, although I do still think they are nicer looking wheels.
After some more emailing with Compomotive in the UK, I've received some better pictures of the wheels that I'll probably be ordering, with a center hub blank for an offset between ET30 and ET40.
This first one is of the proposed front wheel, 18" x 8" with standard silver finish. I'd prefer the spokes not to come flush to the face of the rim, but it doesn't look too bad. I'm also not too keen on having the Compomotive logo stamped on one of the spokes, but again I can live with it, bearing in mind the whole reason I'm after new wheels is not for cosmetic reasons, but to give me some more ground clearance without adversely effecting the handling characteristics of the car.
This second picture shows the 18" x 9" wheel that would be fitted to the rear of the car, this time with optional sparkle silver finish. I like the way the spokes sit further back in this wheel, giving it a bit more or a sporty appearance. The wheel shown has four studs, which I find a bit odd for this size of wheel?
Overall, I think they'll look OK on the Ghibli and after doing some more sums, I'm confident that they'll solve my ground clearance problems. I anticipate well over 20mm additional clearance at the front of the car, which should prevent the anti-roll bar infront of the engine sump from fouling with steep speed bumps.
Just behind the front wheels where the catalysts are located in the center of the car, there should still be about 20mm additional ground clearance with the new wheels. Maybe this doesn't sound like much, but I'm pretty confident that this will prevent about 90% of the fouling I currently get when going over speed bumps. If this turns out to be the case then I might even replace the catalysts with some new ones (since I'm an environmentalist at heart).