Thanks for the input, I guess if anything can do these kinds of miles it's a Lexus.
The one thing, more than any other, which make me a little apprehensive is that the car is straight piped - which is something I really don't like and would have to address immediately.
Does the ISF use the same exhaust / cat system as the 250 or is it bespoke? (I'm assuming so)
Having the entire exhaust system redone would quickly erase most of the savings I'd make going for this one, at which point I'd be better off just spending a few grand extra for a fully stock one on lower miles.
I'm also somewhat tempted by the Cat S one, given how low the mileage is (+ facelift), but my brain is telling me to run for the hills.