The advantage of buying a Lexus secondhand is that you stand a much better chance of it having been looked after properly by the previous owner.
As far as reliability is concerned, mine is now 26 months old, completed 23K, and it is the first car I have owned that has never had to go back for something to be fixed.
The rear seat is Isofix so carrying children is perfectly safe.
Go for it - you know it makes sense :)