One of the first things you need to do is some shopping. And I mean visit a FEW places. Ask them the same questions you're asking here. BUT, do keep in mind you're talking to sales people. By the time you've talked to your 3rd Salesperson, you'll have a LOT more information to work with.
In the meantime, I have some thoughts. 1) I don't know if the Code allows you to vent through your existing chimneys. That's what the Installer has to deal with. 2) I recommend that you DON'T use a ventless unit. Your Mother's age and all tell me her lungs would be sensitive to the by-products of the burning. Granted, you'll hear that these units are 99.95% efficient but, nonetheless, if her lungs are sensitive to ANYTHING, chances are good that her lungs would be sensitive to that .05% of the by-products of the burning! I probably wouldn't be as 'hard' on this point if the unit was only going to be taking the 'chill' off of a room. But since you're CONSIDERING it as a main heat source, I'd get a VENTED unit. 3) See if you can get a unit that DRAWS its air from the OUTSIDE of the house; NOT the inside. By drawing from the inside, it's possible you'll create 'drafts' where the house is 'leaking' air. In other words, if you alread have cold air seeping in in places, you'll only INCREASE that seaping by drawing the air from inside of the house. And on the converse, if the house is tight, where is it going to get fresh air anyways??! 4) Be sure you get a unit that's properly sized for its intended purposes. 5) Think about the 'base' the unit may have to sit on. (Ask the Salespeople about the installation 'process'.) 6) Think about small children being around the unit. 7) Consider a unit with 'blowers' to push the air around. 8) Consider a unit that is NOT a fireplace insert. It's more efficient if the unit is out in the 'open air'.
I'm sure I can go on but you have your hands full. If you have more specific questions, ask away! My best to ya and hope this helps.
PS: They are safe, they are economical. As for fitting into the 'period of the home', there are many styles to choose from. :)