My Insert's Pro's & (mostly) Cons
I started a thread here weeks or a month or so ago about burning coal in my fireplace insert. The folks here gave some GREAT insight & advice; I'd point you to it but I can't find it here anymore.
I've tried using a fireplace insert for the last 2 years. Sort of a similar situation as you: 1966 split-ranch house, not well insulated and pretty drafty, natural gas heat (boiler, baseboard heaters), "mood fireplace" already installed downstairs on the Southwest end of the house in the family room.
I've had some good, but mostly bad experiences as I've experimented with it for heat.
Good: really warms up the family room and adjacent kitchen. "Warm" is an understatement... HOT is more like it. And since the gas furnace's thermostat is in the hall near the family room, the furnace kicks on less, lowering my monthly gas bill by $50 a month.
Bad: The nice heat from the insert doesn't go anywhere except the family room and some adjacent areas. It does nothing at all for most of the rest of the house, unless I set up a series of fans to move the air around the house; even then it doesn't get to the upstairs bedrooms on the other end of the house. As soon as turn on all the fans, they kick up all the cat & dog hairs & dust and such that's been hiding underneath the furniture and in nooks & crannies, and my kid with allergies & asthma has had a horrible time with all the dust constantly flying around the house while we have it all sealed up for winter; plus there's the constant noise of three or four or more fans always running to move the air, plus their electric cords dangling and being in the way. And my electric bill will probably go up because of having several fans going 24/7.
More bad: I bought cheap wood; some was punky, some not seasoned, much of it had dirt or bug larvae or whatever all over it and under the bark. Had to bring it inside to dry out and warm up. And when I did, the air circulating fans took all the plant spores from the molds & mildews and funguses that had been growing on and in the wood (and in the dirt on the wood) and blew THAT in with the already dusty mix blowing all over the house. My allergic kid had an asthma attack and was in constant respiratory inflammation.
My bottom line has been to just use up the wood I've already purchased, and pretty much forget about using the insert from now on. If I can get my hands on good quality wood and then leat it properly season, and keep it covered and dry so I won't have to bring it inside to pre-dry it... well that's be a different story and I could at least use the insert to add some extra heat to the family room/downstairs area.
Just thought I'd add another perspective. Good luck.