Hi Edward, congratulations on your purchase,
I say the warranty offer is not to be missed, these cars are OH so complicated, when problems start no other garage is able to solve them and Lexus will only replace whole units rather than fixing anything meaning they want stupid money to fix any issue.
Just be sure it will be covered for the entire period as they normally stop cover at 6 years from manufacture. It is under written by a insurance company ( not Lexus, just looks that way ) who try and duck out of any claim, usual rubbish like gradual deterioration is not covered.
My 06 plate 450h suffered hybrid problems just after they would no longer cover it, scrapped it!