They're flat bed seats, same as on the 77Ws and some 772s. When the plane is on the ground, these seats tend to go not quite parallel to the ground, because of the angle of the plane in flight. (I've seen DL's 764 seat in a demo at LHR, and it definitely doesn't look flat on the ground, but no one complains about any angle in them in air.) In terms of length, probably best to aim for a bulkhead seat if you can get it, as then you're not tucking in under a seat in front of you. I'm a side sleeper who tends to curl up a bit, so I found the Prestige Sleeper seats great.

KE's headsets are an embarrassment for a country that produces a lot of electronic gadgets. They are NOT noise cancelling and seemed so cheap that I didn't even bother trying them. Even SV provides better headphones (noise cancelling, but some off brand) in J. Definitely bring your own. I don't recall if there's a need for a two-prong adapter or not, so probably a good idea to have one with.