The DOD in a 6.0 is no worse than a DOD in a 5.3. There are plenty out there that have never had it done. We had a lifter and cam replacement due to DOD failure, at about 46K miles while the car was in service. It's been fine since, and now has about 85K miles on the clock. Our Tahoe, though is still running on the OEM cam/lifters with about 125K miles on it now. Take car of the car with recommended oil changes, using the recommended oil, and you should be fine.
The Caprice is an awesome ride! Parts availability is not much worse than any other import. Most of the driveline parts interchange with the Camaro, and the Engine/Trans actually come from the US. I have a Camaro diff and CTS-V brake set to install on our '12 9C1, so yeah, if you pay attention, some domestic parts are available.
They're good, cheap transportation, and fun to drive: