I would say he isnt mad either. When I got my DSE it had 145K on it. It had lots and lots of service history and receipts though. Ive now just passed 170K in mine and its fine, just make sure its serviced properly and change the oil every 6K miles.
£3,500 does seem a little too low for it though, is there definitely no problems with the engine, suspension, gearbox etc??