Depends on the year - the IS300 and the IS350 have very similar engines (both from the same V6 engine family). The main difference is the transmission (6 speed auto vs 8 speed auto) and the trim packages. I believe all of the IS300's ordered by dealers were AWD as I think that's the only way they came from Lexus; it's a mixed bag with the IS350 -- if you live in a warm climate, chances are, the IS350 is RWD, where as if you can find one in a northern state, it's more than likely AWD unless it's from the south or out west. I believe the HP difference between the IS300 and IS350 is about 5 horsepower. Also - it should be noted that the IS350 typically cost more, and I don't think they made as many of them due to the fact that it was the top trim level for the IS (not including the IS-F in this conversation as that's a completely different car).
In this case though -- the one I'm referring to is a well equipped, low mileage IS300.