Oh boy here we go. You know your not going to get a reasonable answer I hope. This is just about the most pointless question to ask right after "What kind of oil should I use".
Alot of Northstars experience the problem, and alot of Northstars dont. You wont find me being so shortsighted that I dont think its something not to be concerned about, but Im also not like the naysayers here who cower in the corner from ever touching another Northstar either, they are some of the best cars in the world and the engine has received award after award for the many positive points it DOES have. A remarkable piece of engineering with a few obvious flaws in the execution.
Trying to quantify numbers is pointless. The best way to look at it is as a problem that you should be AWARE of, but not one that should scare you away. You just have to know what to look for. Dont buy Northstar cars that havent had the cooling system maintained. Thats pretty much it. Once you own it, make sure YOU DO the proper cooling system maintenance as well, and you shouldnt ever experience any problems.
Only you can make the ultimate decision whether you want to give it a shot. I know when I get a chance Im going to get a 2005 STS myself as they are dirt cheap now and pack a ton of features in for the money. As far as your question about the years affected, we havent seen any here after 2003, but the engine design was improved in 2000, with no major changes in 2003... so this is an anomale that isnt quite explainable.
Good luck.