To bring the edge of the tires (edge=end of tread, rounding into the side) flush with the fender extensions, I want to use spacers on the stock wheels.

This setup has been proven to provide (IMO!) the best stance (followed this guideline on various cars, including A4 which sits 'perfect').

How much can be added with 'spacers + extended bolts', and when are 'adapters' (bolt to the hub, wheels bolt to adapter) needed?

I haven't measured yet (does anyone have the dimension?) how much 'space' I need.

 

20mm spacers with 48mm bolts should do the trick....

 

I installed for my stock Allroad double spoke wheels:
20mm hubcentric spacers, 45mm bolts front (worked, no rubbing)
20mm hubcentric spacers, 50mm bolts rear: bolts to long, locking wheel. Ground down to ca. 47mm. Worked, no rubbing.

Result: tire sidewalls not entirely flush with fender edge (probably another 5mm?), but it allows me run a bit lower since the tire edge still tucks inside the fender. If fully flush, it would likely rub on the fender at the current lowered setting.

 

Excellent. Now, get some cheap ECS mudflaps, to complete the wide stance look!