Honest John regularly reports that Toyota Prius Hybrid Batteries often exceed 400,000 miles.
The GS is a wonderful Motor Car.
If you are buying from a Dealer you can always take it to a Lexus Dealer after purchase and ask him to tell you if it has any significant problems. And if he finds any, then you can go back to the First Dealer (within 6 months) and ask him to rectify at his cost in accordance with your rights under the various Sale of Goods Acts.
Good luck Nish and do let us know what you decide.