I think you would want at least a 14 foot model, you will find 12 is too small for your body size.  My brother has an Old Town Otter (now renamed to something else, by the Company) and he is about your size.  It has a semi-open cockpit which is nice for getting in/out and being able to move around a bit and for ventilation.  The tight cockpits can be quite warm under the deck on hot sunny days.  Pay attention to the shape and "lines" of a used or displayed plastic boat...if they are stored crookedly and/or in the hot sun they can deform somewhat.  Even a little bit off of perfect shape can be a constant paddling nuisance...you may have to adjust your steering course more often and crooked kayaks do not track perfectly straight and may be constantly veering off course when you "coast".