If you want an erlier E-Class, it's the general high-mileage things which need to be checked. As you say:
Transmission: check for smooth changes and low noise (whatever the mileage). Walk away from any mechanical noises.
Engine: check for any "clapped-out" noises (tappets, general top-end rattles). The bottom end is generally bullet-proof. Head gasket failure is common at high mileages, check for external oil leaks and water/oil mixing. Check that the viscous fan "tightens-up" after sitting ticking over for 15 minutes or so. Leave the engine ticking over for at least 5 minutes, then rev it up, you should see no blue exhaust smoke. This would indicate valve stem oil seals need replacing. Smoke when you accelerate usually means ring wear, which is not cheap to fix.
Suspension: clonks and rattles are common at high mileages, but are usually fixable cheaply (anti-roll bar rubbers for example cost a tenner).
Rust should not be a problem, but always go for the car with the best bodywork, that's where the real value is established.
Interior is expensive to restore, go for a car that hasn't deteriorated too far to make it worthwhile.
I've no experience with wiring loom or Aircon failures, but these sound expensive. Certainly, make sure the aircon is good and cold, and the engine has no misfires.
It's better to pay a bit more up-front than to keep paying out later.
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike