Water pumps seem to be cheaper here
https://www.autodoc.co.uk/car-parts/oem/1610039436
(I've not used them, simply a google search on the part number 16100-39436)
Also, don't be afraid to ring a Lexus dealer and ask for prices, europarts are not always cheaper than Lexus.
If it's not leaking, or you're not seeing high temps, then you should be fine. Mine is a little noisy but has been for about 3 years!