Hi Mark,
To try and answer a couple of your questions, the wheels my current wheels are the 18" 5 spoke alloys which are genuine audi alloys and the tyres that are on them are 245/40/R18, now the one's i'm looking at as i said are Audi A6 Le Manns 20" they're genuine from what i can tell the part no 4H0 601 025 AB tyres on them are 255/30/R20, they're 5x112's bolt pattern like mine, the rim width on the ones i'm currently looking at is 9J, Offset is ET37 with a centre bore being 66.6mm.
Hope this helps.