Installing a fireplace insert in older home.
Too many questions............this is a DIY forum, so with that in mind..... Fisrt check local codes regarding fireplaces and inserts. What was once Ok may not be now.
Heatilator makes quality products that you can trust. The chimney pipe system may not be compatible with the new insert.
If you have a manifold around the insert for circulating air, a blower improves efficiency.
Although unvented gas ubnits are available, I would always recommend a vented unit. Even if the burner is working perfectly, the combustion process still produces CO2.