Are you looking to be inside Equinix or just in the Lakeside Technology Center? Steadfast has a suite on the 5th floor and could probably accommodate you. I have a box there and their excellent, I highly recommend them.
I prefer to be in the Equinix facillity. We have an ATM backbone managed by another firm in the same location. We're looking to possibly making a move and it would make things easier from a x-connect perspective. Not to mention the ease of getting connectivity to anybody else who is in there.
4-5 racks available in Equinix Chicago? I highly doubt you're going to find that. They've been sold out for a bit, to the best of my knowledge, and during that time, with the demand, I can't imagine anyone in there has much more than 1-3 cabinets available.
Who is the provider you're getting the ATM from? If they're in the building MMR as well that may open up some options for you.
And Alex, I think you had a typo there, we're on the 2nd floor.