As things go, if it's low mile and well cared for you're probably alright. It's going to have Cadillac quirks no matter what. You can forget about head gasket issues and the like, those were all taken care of by 2003. Beyond that, you pretty much nailed it. Window regulators are weak in those cars, but not expensive or hard to replace.
At this age, the coolant tanks are known to crack and split. They're about $90. Not that big a deal to replace.
Depending on what trim level, if it has Magneride the struts may be failing and in need of replacement. OEM is bookoo bucks, aftermarket is available. Personally, I would only consider a DTS. It's the one worth getting, and I'd be very picky about what options mine had if I were looking for one. They came with lots of different options and I would want mine loaded.
Also, don't over pay. They aren't worth that much, honestly. I paid $1,200 for my 2003 STS with only 133k on it. It needed a couple hundred in parts to get it running tip top, and it's got every option available in 2003. However, it isn't perfect. It could still use another 2-300 in repairs and eats oil like crazy. It's getting better, but these engines were babied and they shouldn't have been. It's starting to show 20-odd years later.