While browsing old BMWs I've seen many with a swapped engine. My gut reaction is to avoid them at all cost. But should I? Take the E30 Touring below as an example.
This '89 Touring with a M50B25 from an early 90s 5-series is a car I spotted not too far away. It used to be a 318i.
The swap in this specific car was approved by German inspection authorities.