The Intellijel 7U would be a great choice as it's in the same series and you can still plan on using the 1Us... just note that the Intellijel 1U standard IS NOT the Pulp Logic standard for 1U. So be sure any 1U modules will fit in your specific rack.
With the Mother32, you CAN put it in the rack. But that's something you should really have a good think about as the Mother32 comes in its own case with its own power supply. So putting it in a rack means you lose that many HP for modules that need to be in your rack.
The more you research, the more you'll know. Oh... and always devote some space in your build for adding a few more modules. Even if it's 20HP... the ability to squeeze in that ONE module that would really set things off once you've started USING your system is some good headroom to have.