Full_Throttle

Hello all, This is my first time on this forum and i am after some advise. I am looking to purchase a second hand S5 Coupe probably around the 2008 mark. My question is how many miles is considered a risk with these cars? Are there particular things to look out for? Common faults or things that should have been replaced at certain milages? I am looking at a £17,000 Budget Max! But i would like to include the tax and insurance in that. I have seen plenty that are well within my budget with about 70,000 miles on but there are some right bargain's at 90-150,000 miles. My question is, is 100,000 + a risk? As with all secondhand cars there is a risk no matter if it's 1,000 miles, 10,000 miles or 100,000 miles i am fully aware of that however this is my first V8 And my first Audi and what i don't want to do is jump in at the deep end and a few hundred miles later run into problems. Someone mentioned to me about a "Carbon Clean" could someone explain this a bit further. What are these cars like at 100K do the engines still have plenty of life or am i going to be constantly paying out for expensive repairs? Also what is the servicing cost like on these cars? Appreciate all the help and advise you can hopefully offer. Steve