Hi Mahiddun.
1. It is fair if you are prepared to pay it. They may settle at £13750 if you are prepared to pay that. It is a high mileage ,but if it a Lexus Official Dealer and has a FULL Lexus Service History, it should be ok.
2. A Lexus Dealer should offer a 12 month warranty and may even give you a 3 year Extended Warranty as part of the Deal.Accept both.If you have essentially a 3 year warranty cover, that should give you plenty of time to spot if there is anything wrong. You can then purchase an additional Extended Warranty until the car has covered 140,000 miles or is 10 years old.
3. 150,0000 miles with full Lexus Servicing should be easily achievable.
That`s my view,so let us see what others come along and say.