Mine has just clicked over 210,000 k's without any major dramas.
For the sake of reliability, as well as the belts I would also look at replacing all the rubber components within the cooling system (radiator & heater hoses, etc).
These deteriorate more from age and heat cycles rather than distance travelled.

I think the main issue is that when these engines get up in mileage and they do have a major failure, people normally don't fix them because it's too expensive so we never get to hear what the common issues are with a high mileage engine.

Here are some common issues that you may have already experienced, and if not will likely encounter before long;
Cracked exhaust manifolds on the later models.
Blocked engine breather, but I'd be surprised if any of these were still running the old setup.
Worn suspension bushes.
Torn CV boots.
Cracked radiator overflow tank.
Worn out gear in the transfer case servo motor.
Choked EGR & inlet manifold.
Harmonic balancer.

On the valve clearance issue, I believe they run hydraulic lifters so this shouldn't be an issue.
I would also question timing chain, guides & tensioner(s) but have no experience with this on these engines.

Just on the issue of changing the auto trans fluid, you may want to read up about that on this site.
This has been a hotly debated topic at times, and there are two schools of thought on doing this on a high mileage car.

Injectors are another thing that I would like to investigate myself.
Many diesels need injector replacement/recos at this sort of mileage and mine too is starting to puff a bit of black smoke on acceleration, and I'm wondering if it's due to worn injectors.

I've seen a few E53 diesels around lately that are puffing large amounts of smoke on take off, and from my own experience this has been caused by a cracked exhaust manifold, or chocked EGR & inlet manifold.