Great choice of car - I've not long sold my Avant S4 for an RS4.
There is a decent buyers guide here:
http://www.s4-mtm.com/library.htm
Go right to the bottom to get the buyers guide. There is also a great official Audi techs training manual on the S4 engine etc.
At 62K miles it will need it's timing belt changed at 72K. They typically change the water pump and idler pulley at the same time. It's worth calling an Audi dealership or Audi specialist to establish how much that's going to cost. There's a fair bit of work involved. Basically the entire front end of the car has to come off. Don't be worried its designed to do so!
Check all the disks n pads the S4 has a total of 12 pads! 8 front and 4 back. While not very expensive a complete change of disks n pads is not cheap. Front drive shaft boots maybe worse for wear again not seriously expensive. But it can all add up in your first year of ownership if your going to be doing a mileage that will see you to 72K.
Suspension - particularly the action of the dampers. Like all cars they deteriorate and strictly speaking all cars need there shock replaced by this mileage. Bear in mind this may look like a saloon car but its a serious performance car and capable of very high speeds and acceleration brakes n handling are very important in my book! If you can test drive a couple of S4s particularly low mileage ones with say less than 35K miles. This will give you a feeling for the action of the dampers amongst many other things. Mine were worn at 45K.
There are a tremendous number of front suspension arms which give the S4 it's great steering lock n stiffness for the torque. The lower ball joints should be checked for wear. It's not unknown for one or two to require replacing. If this is the case your better to bite the bullet and buy the front arm kit which comes with every single arm, bush and ball joint for the front of the car at an absolute fraction of the cost of buy the parts individually.
Given that this is a TWIN turbo and the general practice is to remove the entire engine to replace them, which is a big job on these machines. Worn turbos can be a seriously expensive repair. Make sure you know what to look for take a decent mechanic. Even then there are no guarantees. Like all turbo charged engines the turbo should always be warmed up and cooled down. Try to establish this in a discrete way as the owner could simply lie and tell you what you want to hear. Let the owner drive the car out first to see if he warms it up. Let see if he mentions anything about how he has treated the turbos i.e letting them cool down after a spirited drive. Establish what kind of driver he is. If he holds the rpm past 6000 he hasn't got much of a clue. The KKK003 turbo has limited volumetric capacity and there is little point in reving the car past this rpm even tho the red line is well above 6000rpm.
Good luck the S4 is a "wolf in sheep's" clothing.
