I want to install a 300lb chandelier in my living room.

Want to now how to properly secure the chandelier.

What type of construction is in the ceiling? Is there an attic or crawl space above?

No attic space.
I know Im going to have to tear out sheet rock and add some braces.

Just don't know how much to add.

Don't want to tear up more than I have to.

Also, was going to add a 24in-36in medallion so I wouldn't have to sheet rock,tape,float,texture and paint.

A 200 lb person on a 100 lb toilet concentrates weight on the space between braced joists. But I would be equally or more concerned about the fasteners you use to attach the fixture itself, and would envision them giving way first.

Standard fixture boxes are not rated that high. Suggest you go to where you got the unit and inquire or check with a lighting supplier. You doll need a strong structure support, bolted not nailed.

Check out this support at Amazon.com. Should give you an idea of how this all works... ;)

Amazon.com: Westinghouse 0180000 Heavy Chandelier and Fixture Brace: Home Improvement

Mark


PS: There are cheaper versions available online. You could even make this yourself if you are handy! The medallion should cover the hole... no problem!