My advise, based on very expensive experience: If you manage to get it moving on it's own, proceed directly to any dealership other than Lexus and trade it in. Whatever you do, don't put in a Toyota rebuilt transmission. After spending more than $5000 on a certified rebuilt that failed to funtion properly from day one and after 13 months of problems, the dealership here in Calgary (Stampede Toyota/Calgary Lexus) offered me another transmission at at 'slight' discount. Not the Lexus service one expects from this class of vehicle. It was dangerous to be on the road with a tranny which one could not count on engaging when a downshift is expected.
Good luck - I miss the RX300 but I need a car I can have confidence in...