As a first time car buyer, you may want to consider other expenses (other than costly parts/repairs on older model Lexus). What will insurance be like? And with the cost of gas rising daily, can you afford to put premium fuel in a LS that averages 18-23 mpg?
I started out with a used 82 Celica, moved up to a used 88 Celica Turbo 4WD, then a 97 Camry CE and now have a 97 LS400. Best to start out with something that will give you reliable transportation, has a good maintenance record and will fit into your budget. I would have loved to have had a Lexus as my first car, but some things are also worth waiting for :)