The simplest machine that will clean your living room? That'd be the 675. It's currently $219.99 on their website--it can easily clean one room.

But you also mention having them "all over [your] house." In that case, I'd recommend the i7 or s9. I've got the i7 and I think it's fantastic.

There are a few factors to consider, but it's not complicated. Do you have mostly carpet or hard flooring? Any kids or pets? Do you want it to clean the whole house or just one room?