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IMO,the OP should just buy the RX and a Lexus Platimum warranty from Troy for piece of mind.
The RX has very low mileage and he won't find anything close to that RX for $21K.
I looked at 6-7 new crossovers and small SUV's priced from $28K-$34K and found my leased '09 RX with 20K miles for the $27K residual was a much better choice.
Not to flog a dead horse.... but I did not see the mileage.... However, back to a previous comment - I wuold probably myself go with the residual buy+Platinum warranty. But what dealers (should) do in addition to provision of teh warranty with CPO is measure wear on a number of items and replace those that show more than (depending on make -) 40% or 50% wear. This includes brake pads, tires etc.. They also verify various engine measurements, performance of a number of accessories (e,g, A/C etc...) and replace whatever does not work correctly. Sometimes, they will also take care of small dents or wear items such as worn leather arm-rests etc... The real value of this re-conditioning, above the extended warranty value, depends on the current condition of your car, and how sensitive you are to any such items which does not meet the specs. -- whichever way, it seems you have a winner on your hands!
Good luck!