Obviously, the total available thrust will have to exceed the total weight, right? And you need some margin to allow some motors to be throttled back to make it turn / move.
A quick google search for "brushless DC motor propeller thrust" turns up quite a few resources such as http://adamone.rchomepage.com/guide5.htm
Of course, you have the old chicken and egg problem - you don't know how much it will weigh until you pick the motors and the batteries. But you don't know what motor / batteries to pick until you know the weight. Right? So you guess. Pick a weight, see what motors that leads you to, check to see if you are close to your target weight, then guess again. After a few tries, you should get a workable answer.