btw - I used to repair the door assist pumps on W140, not too bad when you have the tools and parts, there is a wiring test for it, whenever there is a fault the system shuts down.
The rear sunshade is a bit tricky to solve but honestly I would ditch it - as it hinders rear view. It usually breaks in Pakistani cars because of the super aggressive "ragra paalish" with rags to impress "saheb" - same story with the buttons you mention.
Your key not returning is a fault of the ignition switch, its broken internally and someone already has tried to mess with it. It is available as spare part - I would suggest using a genuine switch only.
The door cars are rebuildable - you will need new foam and good experience with fiberglas work. The front indicator lens have a catch lock inside which secures the unit to the car, Im pretty sure its broken thats why the "tape"
The wiring is rebuildable (its just wires) - but shahnawaz ltd wont do it - they will sell you the engine wiring loom. A good electricals person can remake the wiring with new wire and reuse the wiring plugs (btw the wiring plugs are also available brand new as spare parts)
The headgasket is not too bad in price - just takes some time compared to a 4 cyl engine. And a good repack requires good sealants too like threebond 1207B, hylomar etc and good tools including 140mm long XZN socket to undo the bolts and a good calibrated torque ratchet.
In all honesty - dont buy this car - it seems to be one of the "aail chenj only" cars and maintenance was "champi paalish with expensive imported stuff" then snapping pictures of it.
Im pretty sure if you drove it - it would have transmission issues as well.
not all W140 have an engine check light - so engine errors are only extracted from the 38 pin connector in the engine fuse box. If yours is one of the models with check light then it may have a "bridge" with some other lamp or even a relay somewhere to fool the driver that everything is A-OK
