I went through this process a couple of years ago and did extensive research on the web. We too decided on using cultured stone and actually designed our floorplan around the fireplace and wood storage. I will post some pictures of our setup to maybe give you some ideas.
You mentioned an insert. Inserts are typically a metal fireboxes that one would put into a masonry fireplace to achieve more heat output. I think what you are wanting is what is typically called a zero clearance unit which is a stand alone unit that can be framed into a wall in close proximity to combustibles. Our unit is a zero clearance unit made by Napoleon.
NZ6000 Wood Burning Fireplace This unit heats our entire house (3600 sq. feet) in southern Indiana with no problem. I put the fan unit in the basement to move the air down and also to minimize the noise. In fact, you cannot hear the fan run unless you turn it up to its highest setting. This unit puts out so much heat that it turns our ceiling fans 15 feet away.
We also consider a unit made by Fireplace Xtrordinair but like the appearance of the Napoleon.
Product Guide - Wood Fireplaces - Fireplace Xtrordinair
As far as flue pipe, I would recommend a solid fill pipe. This type minimizes heat loss and keeps flue temps up. Air cooled pipe cools the pipe which promotes creosote to condense inside the pipe. Your goal is maintaining high temps. Good info here on design
woodheat home
You mentioned airtight units. I would highly recommend an air tight unit. Our unit has two 6" combustion air pipes. All the combustion air comes from outside the house. We lose no heat going up our chimney from the house. Another advantage is that in the summer time you cannot get a smelly downdraft into your house. Our last house we built a large masonry fireplace that was beautiful but very inefficient and smelled in the summer.
These pictures are somewhat dated and taken right as we were moving in. Since I have installed hard maple on the ceilings and finished touching up around the storage door.
You can view the back side of the fireplace here
http://www.tractorbynet.com/forums/projects/114597-bring-more-firewood-easy-way.html