You're right, the GS 450H does have a handy turn of speed
As Krzysztof said, you'll get 40mpg+ on the motorway around 70mph which drops a little the faster you travel. I also seem to be averaging 35mpg tank-tank.
If the car has FDSH then it will have had a hybrid health check each time, hopefully this is in the paperwork or you might be able to call the Lexus dealer and find out.
The lighter colour will hide stone chips better than the darker painted cars.
If both cars are in relatively easy reach, then go take a peek at them both.
Good luck.