:yeah: and even more, I want to expand it to the newer generation of STS either.
I never have owned a sigma platform Cadillac, so I am unable to judge about reliability of these cars. However, as a Cadillac Seville owner, I can securely claim Sevilles are not cheap cars to own in any way you think of, premium gas (Sevilles before 2000), HG problems, active suspension system, oil leaks from half seals and oil pan, and tons of little actuators and sensors here and there that will start to fail, like blend door actuators, etc. When you are buying a 10+ year old Seville with 100k+ miles on it, there is a very high chance that the car comes with these problems, or eventually they will happen during your ownership (I exclude HG problem from this statement, the chance for an individual car is not that much high, although the total number of failed HGs is considerable).
On the other hand, if you buy a newer low millage STS, it will not have that much problems that an old worn out STS has, although the new generation might have more potential problems (based on your claim). The key point is in general, a system with potential problems still has some usable life and if you buy a fairly new car you are still in that life span. Also, since the car is fairly new, the car had warranty or still has, and therefore most of the problems have been addressed under warranty coverage...
However I understand there are tons of people who prefer older STSs over newer sigma platform Cadillacs and that is why they are buying fifth generation Cadillacs. But buying an old STS to save money is not the smartest thing one can do. There are 2002, 2003 STSs with very low mille for very reasonable prices, they could be wonderful choices...