you need to frame out a recess for the display right now so you can tilt it downward without ruining the aesthetic. if you plan for the model of tv now you can get an a precise border with less than 1" gap all around.
secondly, you should plan with your interior designer so the front facade of your fireplace does not have a mantle; you can then mount the television as low as possible. i believe the building code for vented propane fireplaces is a minimum of 12" from the top of the fireplace box before any appliance or built structure, but check with your local builder for state codes. deleting a mantle in this case allows you to lower your television by 6-8". combined with a tilt, any sofa with a seating height of at least 24" will be ideal.