> I think it would be real nice to have a list of these external  factors,
> that installers, wether professionals or self installers
> could go through, to check & recheck.
> To remove as many variables as possible,  to ruining a perfectly good  engine,
> just because of a bad injector, or air bubble or such.

Whatever the others on the list might be, the number-one must-check
and re-check on that list  is definitely the water circulation system,
primarily the hoses. After dropping 3.5 kilodollars on a rebuilt
engine, it'd be just plain crazy to simply hook it up to the old
plumbing.
 All told, I spent about $400 on new hoses for the engine compartment.
After seeing the old ones, I consider it more than just cheap
insurance. My '90 is only 15 years old some of those hoses were
downright frightening. Probably not a bad idea to replace the long
heater hose runs as well, as they run the length of the vehicle,
mostly exposed. A 50' box of 5/8" heavy duty hose is cheap. Never use
cheap slots n' worm drive type hose clamps; sping-type clamps or (if
you're willing to pay the price) aircraft type clamps.
 Check your coolant pipes as well. Excessive rust on the older metal
ones or the metal reinforcement insert squeezing out the ends on the
newer plastic ones is a disaster waiting to happen. On the plastic
ones, check BOTH ends. The engine ends were fine on mine, while the
radiator ends had forced themselves about 70% of the way out. If
they're rusty and questionable, consider throwing  an additional $130
down the rathole to replace the water pipe that goes under the crank
pulley and the flanged elbow it connects to while the engine is out.
It's nearly impossible to jury-rig a fix on one of those if it goes.
 A clogged radiator can cause overheating or sometimes (even worse)
blow a hose off due to excessive pressure. Just like the hoses,
tossing in a new $150 radiator is cheap insurance if you're unsure
about the quality of the one you have. If the PO was a jackass like
mine was, he might have dumped Barr's Stop Leak into the system trying
to stop the heads from leaking, succeeding only in choking the
radiator.
 Check the water pump and thermostat. Bleed the air out properly. 
 If you install  oil pressure and oil temperature sensors and gauges
and watch them, even if something DOES go wrong you'll be able to
catch it significantly earlier than with the "thar she blows!" oil
buzzer and stock water temp gauge alone.

 There are surely other potential pitfalls that could cause a
catastrophic overheat, but my hours of dredging the archives lead me
to believe that the most common failure point is (unsurprisingly) the
cooling system.
"Halten Sie das Wasser im wasserboxer und keine Bange"
(keep the water in the wasserboxer and worry not)

John Bange
'90 Vanagon
"keine Bange, wir holen die Zange!"
(have no fear, we'll get the pliers!)