before buying the car - talk to the owner - if you get replies lke "sirf aail chenj kiya hai, german gaadi hai bohot mazboot hoti hai' - run away as the car would be beat to hell.
the problem you will face is that average mechanics wont have the tools or knowledge base to repair it or even start troubleshooting it - mind you the E36 chassis was not entirely built for small 4 cylinder engines, it shines with the larger inline sixes only, you may find it a bit strange to drive too - the seating position and also the way the power is laid down is not like your average japanese car.
as mentioned - all BMWs with plastic cooling system components are timebombs, they will fail at about the 5 year mark and need to be replaced. Its no biggie and lots of aftermarket stuff is available (not in Pakistan).