2000 Cadillacs are most known for failure of the two Crank Position Sensors. Not an expensive fix, but the car could totally stop running, at any speed, a dangeous problem.
There are two main, very expensive Northstar problems.
One, the head gaskets can fail. This is more of a problem with the 90's motors, but it does continue into the 2000's. A failure causes overheating, massive overheating as the failure becomes worse. The motor must be pulled, heads removed, the head bolt holes drilled out, re-threaded, and a special insert screwed in (The 'Timesert' solution), and then put back together. Somewhere in the $2-3,000 repair range.
Second, a solonid fails in the transmission sometimes. This is the solonid that 'locks up' the transmission for 'overdrive' range. This also requires the motor an tranny to be pulled and the tranny disassembled to get to the solonid. Known as 'The $40 part buried $2,000 deep in the tranmission'. This is not a frequent problem, just an expensive repair.
Overall, any Cadillac has extensive electronics. Which can fail. And be difficult to diagnois and expensive to repair.
Many Cadillac have oil consumption problems. Mainly because old, grey haired people putter around in them, which causes the rings to carbon up and stick, with resulting oil usage. If you take the car out and hammer on it a few times, many times this will loosen up the rings and significantly reduce oil usage.
Cadillac are a dream to drive. Except for the STS and ElDorado 'sports' models, they are not a 'sporty' handling car. But all are very nice to drive. The Northstar is basically a great engine and a great transmission, with only the two above possible problems a concern. You will not wear out a Northstar. If the solonid doesn't break, you won't wear out the tranny either.
As the miles accumulate, there are also what I consider maintenance items that will happen. If the car you are looking at has reasonable milage, this could be nearing 100,000 miles. At this milage, you are going to need to replace the plugs and wires (this model might have on plug coils and no wires to replace), service the transmission fluid, and service the coolant (which should be done sooner than 100,000 miles, in my opinion). The water pumps also leak, the rear leveling system may fail, and in general the struts, shocks, sway bar links, brakes and rotors(probably much lower milage actually) are nearing the end of their life.
You are not buying a Honda or Toyota, which, in general, can achieve 150,000 miles without anything significant happening. If you use a dealer, service is expensive. Of course any 'luxury' car is going to have expensive maintenance and complicated electronics. Mercedes, BMW, etc, etc.
Cadillacs scare me to death. I'm now owning my third. Why? My wife loves them..... What am I to do? I try to own one which is in warranty or purchase and extended warranty.