A Buick Verano (Opel Astra in Europe) would be the quietest car with the best ride quality in your $8,000 range with ~100,000 miles.
The 2009 LaCrosse and up are going to be closer to $10,000 but provide more car for the money compared to the smaller Verano.
The Buick Regal is in between but only slightly bigger than the Verano. Drive all three to find one that fits your needs. Regardless of which model Buick you choose they provide the best ride of newer modeled cars for the price.