well...I'm assuming we're talking W123 sedan here?
Generally, bullet-proof bomb-proof million-machines that rarely if ever go wrong and will outlast most humans, providing you acquire one that's been looked after by an idiot-proof owner.
Not so easy in Pakistan, unfortunately. Plenty of used and abused W123s all over the place, and they survive on. The overwhelming majority are diesel (mainly 200D and 240D with the odd 300D).
These engines are wonderfully reliable and usable everyday. Personally, I'm a petrolhead and therefore not keen on rackety old diesels and manual gears in a Merc.
You'll find mostly very basic spec cars (true of all older Mercs in Pakistan), so no electric gadgets to worry about mostly. Bodies usually fine, but inspect for rust.
One piece of advice...I honestly feel the older W115 generation is better for restoration, mild customization/performance enhacement. It just looks better.