A 550 can be reliable, but it will have needed routine maintenance from day 1 and you will need to continue the routine maintenance. Unfortunately, there are items that will need to be replaced even if routine maintenance was performed. These are things like the valve cover gaskets, alternator bracket gasket, cooling system (radiator, expansion tank, water pump, coolant transfer pipe, etc.) and valve stem seals. Routine maintenance will delay when these items will need to be replace but not prevent it. And there is always the possibility something unexpected will happen and the repair can be very expensive. This could be something as simple as water leaking into the trunk and shorting out the electronics in the bottom of the tire well.
I have owned 3 different BMWs over the last 14 years. First was a 2000 e39 540 I purchased with 38K miles and a CPO warranty. It had no records and I do not believe it had routine maintenance. In the following 3 years, 50K miles, the radiator, expansion tank, water pump, heater core, valley pan gasket and valve cover gaskets were replaced under the CPO warranty. The second BMW was a 2001 e39 540 with 88K miles. It had records showing the routine maintenance. The water pump and radiator were still original. I replaced the valve cover gaskets around 100K and the valley pan gasket around 110K. The current BMW is a 2010 550 I purchased with 62K miles. It had been serviced according to BMW's schedule. In 2 years and 22K miles I have needed to replace the water pump, alternator bracket gasket and valve stem seals. I'm able to do the repair work myself and that is the only reason I purchased this car. Also, there are costs that are not associated with reliability. Gas, insurance, wearable parts like tires, brakes, belts, etc. These all increase the cost of ownership.
Another thing to think about is time. This would not matter if you have someone else work on the car for you, but it does matter if you repair things yourself. I have a second car and it is easy for me to take my time and not rush work on the BMW. It would be more challenging if it was my only car and I had to fix it in a day or a weekend.
If I was looking for a 550 with around 100k miles, first I would take it to a reputable shop for a pre-purchase inspection. I would create a budget of what the PPI shows and what I think would need to be repaired in the next year and start setting money aside. I would also plan on the coolant transfer pipe and valve stem seals needing to be replaced soon and have that money available now. I believe it would be around $1K in parts and maybe $3K in labor from an Indy mechanic. You may be able to find one with routine maintenance and these things may not be need for several years. There are a few forum members who have over 120k-130k miles and have not needed to replace these items yet because they have done their routine maintenance.