I am thinking of purchasing this car and have heard horror stories about Audi’s electrical and transmission issues. This car drives like a DREAM and I have all the service records. It only has 15,500 miles and was used as a “fun” car.
Unfortunately the stories are largely true, so only buy this if you can afford significant repair $$ over the years. The '05 A4 was rated much worse than average, and to that add the more-complex v8 and a convertible. You’ll find a very recent discussion on the problems experienced by an '03 A4 owner here http://community.cartalk.com/posts/list/2119230.page
I suggest you consult the recently published Consumer Reports Annual Car Issue. It has all sorts of information about new and used cars, and you should be able to find reliability data for this car.
Also check the “Used Cars to Avoid” section.
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.
I can’t find an S4 rating at CR, just A4, which is ‘much worse than average’ in '05 for the 4 cyl, ‘average’ for the v6. Only recently does Audi seem to be improving, but I will need to see several years of good ratings before I consider one.
From what I have heard and from the experience of my BIL, they are fine until about 50,000 miles then start costing big bucks every time you turn around.
It it’s a fun car, you won’t put a lot of mileage on it. I wouldn’t worry too much about it. Edmunds.com says that your 5-year repairs will average about $8700. A MB 500CLK or BMW M3 are estimated to have $8300 in repairs over the same period while an Infiniti G35x is expected to have $3500 in repairs. The Cadillac CTS-V is expected to average $6500 for repairs. The S4 is the worst of the bunch, but you won’t drive it every day. You might look at the others to see if you like them. The last 2 aren’t available as convertibles, but they are similar cars and can be used to compare repair costs.
My friend has one, he is “happy” that he has leased one, but can’t wait until it is over. Has mad many issues with electrical stuff and interior noise. Has been towed back to the dealer for breaking down. It had been the ignition system. He is not mechanically inclined so I won’t know the details of most of his problems. They have fixed his problems under the warranty, but for that payment and being stuck on the hwy twice, let just say he won’t buy an Audi for a while. Now this is an N of 1, so maybe he got a lemon.