:) I just put up a newer pic of the cage in your other thread.
To answer: the actual bottom is the same wire as the whole rest of the cage, but with THICK plastic trays set on top. This isn't as hard to clean as one might think, cause the plastic rinses easily. I moved back and forth between aspen and sewn together pieces of repti carpet for the substrate in there.
As far as wire bottoms go, you have to make sure the 'holes' are small enough so the rat isn't doing a high wire act, but large enough so their feet don't get caught. In most cases, I just place a slotted plastic material (called 'Magic Mat' by Oasis) over the wire floor and that does the trick. :)
If the cage you saw on craigslist (which is a great place if you handle it right) really is a ferret cage, it will come with all the trays and shelves too. Make sure to ask for a full list of accessories before committing to it.