Hi,
You are welcome!
I think that 65,000 miles is too low to be suffering the EGR problems mentioned. And 'spirited' use as a hire car initially will, if anything, make the car less likely to gum up the EGR.
But, the recommended service interval for the plugs is 60,000 miles or 6 years, as per this thread:-
So be wary of changing them when a dealer has already covered this in a previous service (before you owned the car?). I don't think the age of the plugs is critical, as that thread mentions.
But, even if the plugs aren't in need of changing, have a look through this thread, it may be very relevant:-
As you probably know, a new genuine Toyota coil, if you were thinking of changing one is probably a fairly hideous expense (~£180?). It is worth considering other brands, I know that NGK are now selling coils on the aftermarket and their prices are as low as a fifth of the genuine part price on some (other) Toyotas.
Also, an unusual size spark plug socket is needed to get the plugs out, it is very thin-walled and not too dear. As per this thread:-
And finally, the cars seem to run better on Denso brand plugs, other owners sometimes report. Which seems a little odd (Denso are part-owned by Toyota, as it happens), but anyway! And the Denso recommendation for your car has relatively recently been changed, I think. It was a 20 heat range and is now a 16, or maybe it was the other way around? The superceded plug is not commonly available anymore, but just so you are aware if you go looking for the Denso plugs.....
It would be interesting to hear what happens next in your investigation!