Well You got "one out of three right"

(1) install a chimney liner
(2) view the fire through a window and
(3) gives little heat output.

#1 you would probably want to seal around the direct vent intake/exaust (a tube within a tube) to keep the cold air out, but check code requirements rather than my suggestion.

#2 Yep ya gotta look thru a window.

#3 If you buy the right one, ---One that is rated as a "heating appliance" they throw out tons of hot air.
We've had the Valor direct vent fireplace for 5 years now.
It heats the whole downstairs of our 2700sq foot house and most the time we have it turned down to less than half full on. (We do live in S Calif thou).

This dummy replaced the force air furnace just before we bought the fireplace, The furnace may have been turn on less than a dozen times in those 5 years and then only to heat the upstairs bathroom.

From all I've read, Ventless fireplace are NOT recommended by a lotta "Pros", and some States/cities--what have you, are outlawing them.

Good luck with whatever decision you make!

Gary