this car is plagued with the crumbling wiring harness problem too, the engine wiring loom actually delaminates and exposes all copper wires, the remedy is to completely rebuild the wiring with new stuff.
Please remember that a W140 is like 3 corollas packed into one, there is an insane amount of parts in the car and all work in beautiful harmony to give that road going yacht feel that this car is famous for.
also remember that this car is old, its a senior citizen now and will have some oddball issues popping up (e.g. steering gear issues, control arm bushing issues, SLS issues. All are repairable if your technician/mechanic is worth his salt and knows how to and is equipped to service these cars (lots of oddball tools)
the M104 engine is extremely smooth but has some problems - one being the headgasket - its not a design failure but plain physics, the head is aluminium and the block is iron - both expand at different rates and the sacrficial item is the headgasket, the failure is usually external leakage or oil in the coolant. - Its almost a recurring repair job every 130,000 kms on these engines. - Make sure the engine is cooled with good quality coolant - otherwise you are looking at expenses of heater valve replacement, heater pump and core replacement too (for removal of core read the next para)
These cars are equipped with dissimilar metal AC evaporators from the factory, they fail due to the same reasons as above, make sure your mechanic knows what he's getting into - removing the dashboard to extract the evaporator takes 8 hours of constant work. The replacement item is all aluminium construction. (good forever)
At this age I can assure you that being in Pakistan this car was obviously one of the "bass aail chenj karte hain - mashAllah baree mazboot gaadi hai" - this literally translates as - All I care is that there is a 3 pointed star on the bonnet and every car drives like a beat up suzuki mehran - this is no different.
reason for above post is - the undercarriage rubber parts would be rotting/shot/torn because "mazboot gaadi syndrome" - make sure you check everything before signing the sale reciept.
the early 722.6 gearbox (which this car would have) - had 2 updates to the TCU which desensitizes the TCU in regards to throwing speed sensor faults, official Shahnawaz imports would have these updates. This gearbox has 2 more issues which show up after quite a while in service and after being super neglected. One is easy to repair - its just a few springs and valves in the valve body, the other is a bit ugly - its the front planetary gear set.
If you hear a metallic racket type noise while moving off in D - the gearbox is going to be toast in a few thousand or a few hundred kms. There are repalcement bushings now available which solve this only if its caught early, putting it off for later (breakdown repair) means that you will need a whole new gearbox as the chewed up planetary set will disperse ground up metal into the gearbox.
Do not assume that a replacement gearbox is easy on this car - if you do choose to replace it make absolutely damn sure the entire model number of the replacement gearbox matches your original. otherwise you will need more parts to make this work - because there are a lot of variations of this gearbox.