The monster one is the 60k not 100k. Some Lexus models need a timing belt change at 100k so he might be getting confused there.
You can take the service plan out at any time. I'd start now if I was you then you probably won't have to pay anything towards the next service this year. It acts just like a bank account so as you have services done the money gets taken out of what's in the account so far. Plus if you sell the car and you've not used up the money it gets returned.
The warranty does require a Lexus service. Taking out a warranty on a 10 year old car I don't blame them.