I checked into struts for a friends LeSabre and they were quite expensive. I'm sure that you know that there are struts at all four corners on those beasts.

I wouldn't think they need replacing unless the car bounces a lot with no rebound control. With good struts the car will still feel like boat compared to a Benz.

I couldn't say for sure about the KYB's for the Buick, but KYB's for a Benz are REALLY stiff.

Good luck,