Just a heads up, I purchased an 07 GS350 for my wife, and had an oil consuming issue as well. I was devastated at first, but did some intel. Stopped at a local Lexus Service Center and gave a tech $20 for a few minutes to talk. He told me there were def oil consumption issues with the GS300's. My issue turned out to be the PCV valve. I was using 2qts. per 1000 miles. Since I changed the PCV valve and added Lucas Oil Treatment, (knock on wood) it's gone almost 3000 miles and no oil loss. AND for price comparison for you:
I bought my 2 owner car with 104k for $13,500. They were asking $14,900.00 but they're definitely out there. I passed on a black one that had 94k, as I preferred the silver one (came with 2 keys and 2 sets of floor mats). But you could definitely get the 350 vs. 300 for the 13k range.
Good Luck!