Used cars at dealers will always be in good condition. I believe you are required to service the car regularly to maintain the warranty, and used cars that still have a remaining warranty can be extended up to two years by being "mclaren certified" for the price of $3500/year.

The risk is that you've bought a low mileage, out of warranty car where issues that will probably come up, haven't. My car had a million things that needed to get fixed and it had about 7500 miles when the warranty expired. If you buy a 2K mile car out of warranty, I think you may be asking for a headache. That is on 12Cs though. Can't comment on 650S and later builds.

As always though, low mileage, out of warranty cars mean they haven't been used enough to spring leaks and find loose clamps etc.