I realize that a Lexus is a toyota, so it should be reliable, but how many miles could I get out of a 1999 Rx300? 150K? less?

On average, it is recommended that a car should not go more than 20,000 KM per year. Anything more than that is too much milage. Lexuses are excellent cars, they do not break easily. Make sure that the car you want to buy has been through regular maintainence with the agent (Toyota guarage). You can find out at their office by giving them the registration number. I have a 2002 RX300. It is still running as the day I bought it. Go for it.