5x100 is the PCD for all Mk4s, with a 57.1mm centre bore.
Offset depends at least partially on the width of the wheels - somewhere between +35 and +38 is normal in the 6.5" to 7.5" width range.
225/40R18 will maintain your aspect ratio (stock fitting on the R32 18" Aristos), but will make the car ride quite hard. I personally find 17" wheels to be a good compromise, I'm currently running 235/45R17 tyres (225/45R17 is stock on 17" wheels on the Mk4, but 235/45R17 tyres are considerably cheaper).
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