I have seen the older Explorers with 300k, (96-01 V8) but they are known for transmission failures long before the engine goes. As for the Lexus RX, I have owned several of the RX 300, and seen those with nearly 400k and still running ok. It really depends on the maintenance, a lot of that now a days is listed in a CarFax Report, if done at a reputable shop. The only problems I see with newer RX350's is not so much the engine or trans, but electrical gremlins. Good luck with your new Lexus!