I'm making a belly cake, can anyone tell me if 1 batch of mmf is enough to cover it or not and if I can decorate the cake the night before without the spoiling the fondant.

It depends on the size of your belly cake, how thick you roll your fondant, etc.

You might be able to use a chart to estimate:
http://www.satinfinefoods.com/product.htm

It depends on how big the cake would be. Global Sugar Art posted a helpful guide: http://www.globalsugarart.com/?id=24

You can definitely decorate the night before.

Good luck.