Is rebuilding a engine in a bmw 750li more effective

Rebuilding is probably better than buying a used engine and checking it. As BMW owners all tend to drive like assholes, any engine you find is likely junk. And a new engine will cost more than the car's worth.

It’s 19 years old and an XDrive, not worth putting any money into it.

Used engines are the most cost effective solution. Try to find one from a reputable source (many junkyards list on eBay and car-part . com), even better with a warranty. Rebuilding a modern engine properly will be extremely expensive.

Money wise, it would be cheaper to find a used engine if your current engine is toast and or overheated.