Re: Alloys question - offesets, PCd and centre bore
Centrebore would have to be the same or bigger (and use spigot rings)
If you really wanted to you could use wobbly bolts to accommodate wheels with a pcd of 108 or 112, if you really, really wanted something much different you could have some adaptor plates made. but then its all about the relationship between the brake caliper, the suspension struts, the wheel arch and the hub face. I understand that you can get away with 5mm spacers on standard 18" wheels so therefore an offset of 35mm on 71/2" rims works.
The modern vauxhalls with 110mm pcd have the right centrebore, pcd and offset to be accommodated. But to be honest, even the big irmscher wheels are boring and overpriced.
However, the thought of perhaps 20" wheels being kept central by a bit of plastic with bolts that allow the wheel a couple of mm play with 200bhp+ scares me!