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Horses for courses!
Buying from an independent dealer will be cheaper but the warranty, as opposed to a Lexus Dealership, will not be as good. Go with a car that has a full Lexus service history if you buy from an independent. Look at condition rather than mileage, provided it has full Lexus history. Buying from a Lexus dealer is the "safe" option but you will pay a premium for peace of mind. Also you can purchase an extended 2 year warranty (in addition to the first year warranty) provided you have it serviced at a Lexus dealer. Naturally you will pay a premium but you can get service plan that works out cheaper, which you pay monthly.
The age of car you are considering, I would go with a Lexus Dealer with the above benefits. You will know your outgoings from Day 1. They are very reliable cars but some repairs can be very expensive. It just depends if you are willing to take the financial risk yourself or have the comfort of a good warranty. A full Lexus service history is well worth the extra you will pay, even from an independent dealer, in my opinion.
Good luck with your search and please let us know how you get on.