You should post your question on the discussion forum at hearth.com; that's where you'll find more active discussions about fireplaces and fireplace inserts. You'll get more useful answers if you give measurements of the inside of your existing fireplace, and better yet post a photo. For an early colonial look you might want to focus on inserts made of cast iron or soapstone. People at hearth.com can steer you to particular brands. Here is a link that might be useful: hearth.com

So you currently have an interior chimney w/ a fireplace open on only one side & would like to possibly convert that to a two-sided unit that would serve both the living room & the adjoining room being remodeled? Intriguing idea, but if you want to have a gas unit, I'm not sure there is a two-sided insert available to put in an existing masonry fireplace, if that's what you have. There are two-sided gas fireplaces (sealed-type heating units, not just a gas log set) for new construction. Your best bet might be to visit a few dealers to see what's available in your area. Have a couple of them evaluate your existing fireplace & its condition. They could advise you on what's structurally & economically feasible. Might make more sense to put an insert in the original FP & add a new FP or freestanding stove in the remodeled room. If the original FP is not masonry but is an older prefab type, maybe it could be ripped out & replaced w/ a two-sided gas unit.

Everything you are feeling is very normal and will pass. After a year or so when you are done all these thoughts will have mostly faded away and you will have a lovely home to enjoy. I am saying this to you as fact and a reminder to self...I'm growing weary of doing all planning, designing, ordering and everything except heavy ape work and have started delegating to save whats left of my head. I do know this burnt out will pass though as I have been through remodeling many times before. Have faith in yourself and trust yourself...you can do this!

I had the same thought as Dan - have them basically rough in the fireplace for future install without actually putting it in. Then have one put in on your own after you take possession. Costs are so dependent on where you are, it's hard for us to tell you what the cost "should" be. For reference, our cost to to put in a corner (glass on two sides) gas fireplace with stone surround up to the ceiling is just over $7,000 (that includes the fireplace, the install and necessary gas/venting, the stone, and the labor to install the tile). So if you're talking $6,900 total, it sounds pretty reasonable; if it's that on top of the cost of their basic fireplace, I might ask more questions (but it still could be reasonable).