Off the top of my head it's a 35mm offset, but don't quote me.
If you want to run 18s and are not worried about the whole defect thing, to get as close to the original overall diameter you'd be looking at running 35 series tyres. In terms of width, anything wider than 7" and you will start to have clearance issues.
Check out the ST162 dedicated thread and ask the guys on there what wheel/tyre combos they're running for some ideas.