Moving upto 20's

[Davie b]

Hi guys ,
Just bought a 98 S8 PF , I want to get a set of 20" BBS LM Replicas fitted ASAP , a wheel manufacturer called "Dare" do a set that look pretty good .

Now I want them staggered at the back , so can anyone fill me in with what size wheels and tyres i need to go for , baring in mind I have absolutely no understanding of offsets and that lot , so any explanations would be appreciated.
I understand that I may also need spacers so any links as to what to get would be great ,
Cheers
Dave