Greetings folks,
I've been lurking for awhile and this seems to be a very helpful group. I am contemplating purchasing a 2001 S4 with 79k miles for about $20,000.00 I am new to the Audi brand, but just keep hearing great things particularly about the S4. I have been a BMW loyalist for about 12 years (528e, MCoupe, M Roadster)

So here are my newbie questions:
1.Are all S4's bi-turbo?
2.Given the year and mileage, are there things I should check during physical inspection (it seems the TBB's are a somewhat common issue).
3. The S4 I am looking at is not from an Audi Dealer and therefore not Audi pre-owned certified, is this something I should be concerned about?

Answers to the above should get me pointed in the right direction, but please feel free to add additional pearls of wisdom.

Thanks in advance for your help.