Congratulations on your new pre-owned Lexus. You'll love it.
The answer to your question is yes & no. In earlier years, even if the car had the "Sat" button on the dash, if it didn't come equiped with the satellite reciever as part of the original factory package, you would have to have the dealer install it at your cost.
With the newer models, it is my understanding, that most models come "Sat" equiped ( ready for XM) but you must contact XM online, (there should be instructions in your owners manual.) When you talk to XM, they will give you a autorization code that is specific to your radio. You have to enter this code into your radio using the key-pad. Once you have done this, it activates your account with XM, the two units talk back and forth for a sencond, and your done.
I love XM radio, and I don't think it's too exspensive
I hope this has been some help.
Paul