Dont want to be a scaremonger, but I was seriously considering a 996 c4s until I saw this !
With my luck of engine failure
....thought otherwise.
Although I suspect you have done your reseach on 996 issues
Found this on Total 911
"All 996 engines have an intermediate shaft that runs the length of the underside of the block and is chain-driven from the crankshaft. This, in turn, drives the twin camshafts on each bank of cylinders. At the end of this is a sprocket which turns on a bearing. The sprocket is retained by a small stud that can break, causing the bearing to fail. At first, this manifests itself as a noisy rattle, which turns into a death rattle as the cam chain come off, leading to further internal damage."
Also found this else but cant find web page...from memory
Basically regarding same sealed bearing, which says "ims" bearing will possibly run dry over a time, causing bearing breakup resulting in major engine failure, especially low milage 996's which have been driven carefully, but 996's which have been driven harder(trackdays etc) will get engine splash which coats the bearing in oil which will prolong engine life.