[Phaeton]

So because the engine swap idea kind of fell through - mainly due to lack of knowledge on my part - I'm just going to do the fall back option of maintenance.

Here's what's in my ECSTuning cart right now - along with my reasoning for it:
1. Timing Chain kit - I'm past 150k mi
2. Valve Cover Gasket - dealer said it was bad
3. Water Pipe Kit (thermostat housing too)
4. Spark Plugs
5. Thermostat - it'd feel weird to replace the thermostat housing and put the old thermostat back in
6. Coolant temperature Sensor (green top) - van was having problems starting while hot
7. Intermediate Timing Chain Gear - $117 from ECS - wish I could just have it 3D printed!
8. Serpentine Belt - this was done 50k mi ago - but if the engine has to come out for the timing chain anyway
9. Water Pump - not sure this is necessary

What am I missing? Should I do the head gasket while everything is open? If the priority is the valve cover gasket and spark plugs, does it make sense to do the rest? The van throws two codes P0171 and P0411.

[robinson1509]

Test out your front o2, your combi valve and secondary air pump. If you do a compression test ( http://www.gruvenparts.com/12v-vr6-compression-test-instructions/ ) before you remove your engine it will help you decide what to do with the head. It is a big job, I would tear it down and do mains, rings, valves... If you do the work it will still cost 2 or 3k in parts to do a complete overhaul but you would be paying more than double that just for a chain job. You will find lots of things to buy when you get it torn down. Definitely having a complete gasket set is a must either way. Check out this page for some inspiration   http://www.gruvenparts.com/12v-vr6-timing-chain-and-guides-replacement/   

Is it a 12 or 24v ?

[Phaeton]

Its 24V.

At 150k mi should I do a complete rebuild or wait longer?

[Itsamoto]

Seemingly, because there is no defined service interval in the Bentley (or any documentation that I've seen) for the 24v timing chain -- the consensus from Mk4 owners seems to be that you run until such time as it either throws a related-code or rattling chain noise develops.

At 150k miles, you are at start of that window. A VW mechanic that I trust whole heartedly said the chain system on the 24v VR6 should be good for at least 300k km if proper oil is maintained.

From what I saw of my engine's chains at 250k miles, the guides showed deep grooving -- but grooves themselves are normal - but the oil passages were starting to gum up with residue/sludge. So i think this is evidence that maintaining clean engine oil is super critical to the chains being properly lubricated and the chain tensioner working properly.

[Phaeton]

What's a related code? Via VCDS I'm aware that there are misfires on at least one cylinder. I don't have an autoscan on me, and the last autoscan was months ago. Oil changes are done about every 5000 miles.

[GotMojo]

Shop around.  You can do much, much better than ECS's prices.

Check your alternator.  Mine went at 115k.
You might as well replace the ball joints and tie rod ends.
All the bushings are probably shot as well.
Get some silicone hose and replace every rubber and/or cloth hose that still exists in your engine. Even the 90 degree bends.
Yellow oil filler tube.  (it will snap in two)
Cam sensor seals
Cranskshaft seals
Intake gasket
Valve cover seals (the rings for the spark plug holes)
Inspect the water pump.  I would just replace both of them if you're going all-out.  O-rings at the very least
Coolant temp sensor
While you have them out, check the radiator fans.  They have a tendency to disintegrate at 100k-120k miles.
REINZOSIL
Hi-temp paint (For the valve cover.  You do want it looking fresh, Riiighhht???)
Oh, and bolts. You're gonna need bolts.   Lots of bolts.  VW uses the stretch variety.  Don't reuse the old ones if it says 'must replace' in the Bentley.

At the very least:

Axle bolts (12 total)
Thermostat bolts (you'll probably strip them)
Ball joint bolts
Flywheel bolts (not sure about these)

There are a few more that I can't remember at the moment.

Save yourself some work.  Before removing the engine, take off the valve cover and get a camera down into the lower chain area.  You should be able to see the lower sprocket and (hopefully) determine if the guides are broken, the sprocket is showing signs of wear, etc.  You might be able to save yourself a lot of work.  I fully expected to see a black gummy mess but my engine was very clean.  Full tear-down averted!