What should I look for when purchasing a 2013 S550 with 67k miles? Does the car have trouble free life left? What issues become apparent with this age and mileage?

Not knowing the service history or how it's been driven, L&R front struts are possibly due for R&R. As the car gets closer to 100,000, L&R ball joint/ A arm will need R&R and also motor mounts and transmission mount.

Get a good after market waranty and the above should be covered.

Good luck.

How about the transmission?

Knock on wood mine mine has been 100% trouble free.