I know two people who own that car both 2005 S4s. One convertible and one hardtop. They both have exceptional reliability with the vehicle. I believe Consumer Reports has average or even above average reliability for the S4 which differs from the A4.
I would take the car to an (pay) independent mechanic for checkover. They will fill you on maintenance/repair costs which are much higher than average.
Even the worst reliability car listed by Consumer reports has about 30% of vehicles with a problem. The balance do not. So essentially 2 in 3 have nothing go wrong.
While reliability is a factor and being cognizant of it is good. It is not the primary factor in vehicle ownership. It seems to be a bragging right of many otherwise boring vehicles.