I woke up this morning to a flat right rear tire on my '99 528. It turns out the flat was caused by a screw, but when I was putting the spare on, I noticed that the flat looked like some one had taken a knife a cut a chunk out of the side of the tire (the inside side). I figure this would be a considered a blowout risk and is probably worth replacing.

The tires were bought 9/2004 and have about 19,000 miles on them. All four tires are Kumho ECSTA ASX's. The fronts are 235-40-R17 (94w) and the rears are 255-40-17.

Here is the million dollar question. Should I just buy one replacement 255-40-17, or should I get a new set of four? If I get a new set, should I get staggered or non-staggered (and do I have a choice without replacing the wheels)? Finally, if I get 4, are the Kumho's a good choice, or should I be looking elsewhere (in regards to my needs: it snows once a year here, it rains a good bit, and I don't drive the car very hard - most of all, I want quiet).

Sorry if this seems like a dumb question, but I don't know much about tires.

Wad