Brake caliper clearance
 

Given that my S8's transmission is now dead and uneconomical to repair, I'm going to be breaking my car for spares.

I want to take off the wheels ASAP, and replace them with something cheap for one or two short drives - my question is, what am I looking for in a replacement in terms of size etc? I'm assuming there aren't going to be any steel wheels suitable?

Depends on what your car is. If it's an S8 with Brembo front calipers, then you'll have a very limited choice I think

The manual for the S8 mentions 17" winter wheels (vice the normal 18" Avus) and IIRC the OEM winter wheels are steel - tho' i might be wrong there.

I would leave the wheels on it until you're ready to make it immovable. Most scrap metal places will happily lift it on arrival for you to remove the wheels - it's what my dad did with his A4 2.4 when it died