to retain or upgrade to GX550?
Go for it… 
You have been redirected to the Car Talk forum, so we have no idea why you are asking your question, nor do we know what year GX550 you are thinking of buying…
Sure,why not?
Lack of confidence in the engine would be a major reason, there have been engine failures in Lexus vehicles since first introduced in the LS500, and LX600.
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In that case, just trade the vehicle in before the power train warranty expires. Me? I have no desire for a $75K Lexus, I would much rather have a Bronco.
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New cars are expensive. If you can afford it, then do it.
14 million new cars were sold last year.
You bought it 10 yrs ago. I assume you didn’t have a 10 yr loan. You saved cash to buy new car?
How many miles in your GX450? What else might you use the money for? If 11 years is long enough for you, buy the GX550.