Luxury cars depreciate VERY quickly once they are out of warranty because of how expensive they become to maintain and repair.
Take a Cadillac and a.. Cobalt.
Cobalt new = 20,000
STS new = 60,000 - 70,000
Every part in that cadillac is more expensive. The tranny on the STS might be 5 grand, they can't put a 5 grand tranny in a cobalt and keep the price at 20 grand to sell. so the cobalt gets a $1,200 trany
So when they both get old, and you need a new transmission, your either going to have to buy a $5,000 transmission or a $1,200 transmission.
Now i'm not saying the tranny is going to go on this thing, that's just a comparisson. But the maintenance cost of EVERYTHING on the car will be more. So if your ok with that, then ya, buy the car. if you don't want to worry much about the cost of repairs, get something that is a good reliable inexpensive car to maintain.
I would suggest for the price your going to be putting out, move up a year maybe two and get something with some warranty still on it.
2006 was the last year GM had their crappy 60K powertrain warranty.
Get even one year newer and you'll have a better warranty on the powertrain. Mind you for that price you could get into a brand new Impala, or G6. (But without all the toys)