I think the R is the perfect car, and definitely one of the best you can buy for the money.
I've never driven a B8 S4 (or S5), but have driven a B9 A4. While the car is definitely super comfortable and the higher trims/packages have all the fun technology, it just felt too big to me. I don't see the S4 feeling any different, since it's the same size.
The 8V S3 is pretty much the same thing as the R. Buddy has one, and it's a great little car. Personally I don't like the interior and the way the Audi MMI works, but that's personal preference.
Absolutely no experience with an AP2 S2K or anything even similar. But I lump any 15 - 20 year old "performance" car in the same bucket as Rice's B5. A fun weekend/project car, but not something you want to have to rely on as a reliable daily. Just the nature of the beast. If this car isn't meant to be your daily, the have at it.
tl;dr: Get an R, even if you don't buy mine