We just put DS1 into the Kura a few weeks ago and have his mattress at floor level, however we haven't yet put the slats in the bed and wont until we 'flip it over' and the mattress will then be 30cm or so off the floor. The bed is completely stable without the slats and as we currently have DS on the floor until he is a bit older, he can stand up on the mattress and not hit his head - AND see through to the canopy which is cool as we have one of those xmas rope lights under the canopy which he loves loves loves. Once he is much older, he can sleep in the top part and we will put all his toys etc underneath for more playspace.
So what you can do is place the bed slats on the floor and then the mattress on top of them and the 'voila' you have a supported mattress at floor level. I can send anyone a pic if you want to see how we set it up (just PM me )
And another handy hint, I bumped my head on the bed a few times so bought one of those pool noodles, but a slit all the way down it and put it on the bed rail at head hight - its a great bumper bar - cost $3 and as I got a blue one it looks cool
HTH
Naomi