A Moby 50V is the most comfy moped you can get for a reasonable amount of money, but it is French so expect to get a baffled and a bit frustrated when tinkering around with it.
I'll second the recommendation of a Free Spirit, see if you can a silver one for $600-$700 - then convert the rear wheel bearings to sealed units. I'm 6', 2" and they fit me well enough but might be just a little small for you; get creative with repositioning the seat.