I just bought a 1 owner, Cadillac Certified, 2006 STS-V with 20K miles and new tires with a clean Carfax for $32K. It was listed at $36k. I looked for a month and a half and finally found mine. For me personally I wanted a low miles, Raven Black or Thunder Gray, non-smokers, clean and non crashed car for under $35k.
As for things to look for. I found in my searches that most of these cars will have terrible curb rash on the wheels. So depending on how picky you are, you can try to find one with good wheels or just budget to get chrome ones later. Most of these cars seem to be in the East half of the country. So if you're in the west like I am, it makes it a little tougher. Good Luck.