I would not buy a used BMW with over 50,000 miles unless the seller could provide documentation showing oil changes no less than every 7500 miles.
A 550 with over 100,000 miles has very likely had only 6, maybe 7 oil changes in it's life. My 550 has 90,000 miles and has had 18 oil changes in it's life.
Walk away. It's likely a money pit.