The plastic timing chain guides are bad, most of the issues come from that, they get hot and get brittle and start breaking between apart around 100,000 and 150,000 miles, there are a few outliers that go more but most will start to have issues with start up rattle and progressively get worse and it seems to have no bearing on how well or poorly the engine was cared for, plastic like that just don't belong in an engine. The only time I have seen bottom end issues is when there are plastic bits plugging the oil intake. When the chain noise gets real bad most shops will diagnose a bad engine. The head gaskets and head surface seem to be very susceptible to overheating, if you have a water leak FIX IT, any overheating will blow a head gasket and probably need head re-surfacing, most shops diagnose this as a bad engine. Sometimes after sitting a while (usually a week or more but ocasionally just overnight) a lifter will bleed down and allow the rocker to fall off causing weird noises and severe misfires that most shops will diagnose as a "bad engine", typically we can just re-install the rocker in an hour and the engine keeps going. We see a lot of people bring their rigs to us with a "bad engine" and it does not need a new engine. We offer timing chains as a menu item here with all guides, tensioners and chains for $750 and if you want head gaskets at the same time $1500 plus $150 if you need them re-surfaced, most shops refuse to dig into one of these at all and most shops in the valley send their customers to us because you can't find a "good 4.7" in a junk yard.