As far as putting a 3k tv above a fire place that might be used... I wouldn't high heat can screw with anything... and if you don't get to buy toys very often, why risk it...
I'm sure it would look very pretty, but are you willing to take the risk??? If you are insistant on doing it... maybe try putting a couple of different sized candles in the area where you plan to mount... one stubby, one skinny, etc... and fire it up and see what happens...
I'm a projector person and don't really care for huge tv's that are smaller and cost way more than a projector setup... however projectors are a lil more tempremental, but problems can be avoided by setting it up in certain ways... however, every set up is a lil different... you really got to get dirty with the details of your situation... when I got started I did a lot of research in places like here, avsforum, projetorpeople, projectorcentral, etc...
The devil is in the details... do your homework and make sure its pretty solid... and ask a lot of questions...
If there is anything I can do don't be afraid to ask... Good Luck