ventless fireplace

I don't have answers to most of your questions but I can say that ventless gas fireplaces will create a LOT of moisture in your environment. This can cause mold. As far as health is concerned, you will need to have a good working CO detector.

Depends on what kind you get and what fuel is burned. Some gel fuels only expel wter, which means yoi may want to keep and eye on the surrounding area and towel up after a good use. I have one that only creates water waste and I haven't even had a moisture issue, but I don't run it hours on end and it's mire for the nice ambiance than to make the room warm.

About costs, they have standalones that cost as cheap as a few hundred and some custom installed into walls, etc. that run into the thousands.

Tx for the infos.

I guess i would have to get a dehumidifer possibly.

Would you suggest the gel over gas /butane versions?

is it a DIY job or something you have to hire a licensed contractor for?

Completely dependent on the unit you buy. Better to ask the retailer.

>snuffles- are you planning to do this in a single family house? An apartment? Can you tell us the approximate location so I can be sure to be nowhere nearby?

Your questions indicate a naivete/basic unconcern about installation of what might be hazardous to others (i.e. nyc laws, installation,fuel, etc.).
Have you even seen the recent problems with gels?

You should check the building codes. NYC has very stringent restrictions on mechanical equipment. I'm pretty sure that a vent-less fireplace will be an issue. you should hire an engineer with residential experience.