I agree w/Dodgeman on this one. More and more lately, I'm reading how these ventless fireplaces are giving people breathing problems especially if their lungs are sensitive to 'irritation'. You know, asthma, bronchitus, allergies (yes allergies). ANd for 'older folks', they're not too healthy. I would consider a ventless unit if it was only used to take the 'chill' off of a room, and it was only used a little bit, and if there was only 1 unit installed. If you're planning on multiple units, ventless could be disasterous so to speak. Since you already have stacks for the coal fireplaces, in the most common situations, they can be easily adapted to be used with vents. (The industry has come a looooong way in this regard.) To boot, the contractor does have to follow Code when doing the installation.
Maybe others will have more to say. My best to ya and hope this helps.