yea....you do need an overflow...thats what i had...a glass sump with bio media. ....I also had a bin as a sump, and a media filled container on top of the bin like you said. works out cheap this way.
you are right about chances of water draining from sump on the floor if your pumps fails or overflow outlet fails...but i rarely have this happen. :)only once though...as i have poured too much water into the sump..but i am quick enough to recover it.