FWIW, I had two used cars previously that were stick shifts and didn't have any problems with them.
Besides, I will not be able to buy a S4 new at $50k+, but 2-3 years old used, I might be able to do it.
How long did you keep those used cars after buying them used? I realize it's hard to gauge the prior owner of a car and there are many variables at play. Even though I found nothing wrong with the used cars before buying them, I still had problems after a year of owning them.
I also wouldn't be able to buy a new S4, but I'd be seriously hesitant to buying one of those used with a manual. I'd guess a typical owner of an S4 is likely to be pushing it hard (especially if they went with the manual tranny). Otherwise, why else would you buy a performance oriented car like that?