I too have spent the last couple of months looking for a RX450 at about £15k - - the 'red lines' for me were that it had to have a full Lexus service history, bodywork in good condition, wheels unscuffed and interior clean.
I didn't think that would be a problem until I started looking, and it's amazing how many fell outside those parameters
Few Lexus dealerships sell cars of that value (though some do - and they are worth looking out)
Whilst I guess you may already do this - I also checked out the Gov.uk MOT website to look at the mot history of the cars I was considering (which gives an impression of how well they may have been maintained (and even more useful with older cars) and I always use the Parkers Price Guide to get a reasonable indication of the pricing ration between year/mileage. It cost me a tenner - but has saved me a great deal of time (and cash))
Though it may well be of no surpise at all - - nearly all of the Lexus dealerships work at the upper end of the pricing (as do some of the independents tbh), but there are reasonable examples out there at reasonable / fair prices.
I was certainly not concerned at high mileage cars (proving FLSH of course) - for the reputation of Lexus reliability and quality precedes it, but I was careful to check for bodywork damage and wheel condition. (Though if sold by a Lexus Dealership these should be good - apart from one I saw - which is still for sale on a Lexus forecourt with two areas of damage and rust that the salesperson "didn't know about"!)
Enjoy your search Ben, (I have) - and I've learnt a great deal about the marque and the RX450 models - -
My search has come to an end - I think - and subject to a proper check-over/test drive etc etc, I too will joing the ranks of Lexus owners in the next few days.....