If the car is ON and showing READY on the dashboard - then stop/start traffic will not drain the battery. If you have turned the car to accessory mode, it will drain the battery but it'll turn off before the battery is completely drained.
MPG depends on the driver - no 2 ways about it - people can claim to drive two different cars in exactly the same way but chances are they wont. For example, I drive my RX450h in a more relaxed nature to my wife's IS300h as it encourages more hard cornering etc
Not really many issues TBH, these cars are bulletproof. Just do the usual checks, make sure there's a full service history, no crash damage and you should be fine. The leather seats are VERY soft so look out for any scuffs and marks. Kerbed wheels are annoying to get back to the original colour so ensure all 4 are the same colour. Apart from that, these don't really have any issues