As far as mileage goes, that motor is pretty bullet-proof. If the previous owner did even the minimum for service at the recommended intervals, it should outlast you. A current high mileage Ninja owner might be able to alert you to any trouble spots, if there are any.
As far as brakes; check for even wear pattern and pad thickness. The brake disk(s) should show the same "wear" pattern all the way around the disk, and there shouldn't be any deep grooves dug into the disk. They actually should look very similar to your cars brake disks. They will show wear, but nothing uneven or nothing deep enough to catch a fingernail in if you drag your nail across the disk. Also, check the brake fluid level. If a previous owner says they have kept up a good service interval or schedule, the fluid should be full and relatively clean.
Other tell-tail signs of service (or lack of) would be simple things like tire pressure and sprocket/chain wear. If they don't even keep near required pressure in the tires, or keep the chain adjusted and lubed, why would you think they went to the hassle of changing oil.
Just my thought's on what to look for.
