The Hybrid Warranty (assuming it passes) goes forward and has no regard to past mileages. 10,000 miles annually each forthcoming year.
I would only buy from an authorised Lexus Dealer as they provide a comprehensive 12 month Lexus warranty.
Have you checked the vehicle history via the DVLA website ? It provides an invaluable insight to the things that have needed attention to pass the MOT Test
Timing chains can last 200,000 miles as has the one on my Merc. I do not know of Lexus` experience. Ask Lexus Chester or whichever Dealer is local to you.
I have a 2010 GS 450h, which I have had for 6 years. It has covered 60,000 without a murmer.