I'm a heavier rider than you and would say a minimum engine size of a mountain sled is 700 but 800 is preferable. The Edge chassis you are familiar with is a good mountain sled and very reasonably priced. I'm not sure where you plan to ride but I'd say a minimum track length of 151" and prefer a 156" or 159" length. Lastly, even though that generation steering posts were not designed strength-wise for lengthening, a good aftermarket 4-6" extended length and higher strength steering post (wildchild on this forum has built me several quality steering posts). Also you'll have to extend the throttle cable and brake line will allow you to stand up straighter.
Hammered