No problems about the extra posts.
The USB's won't be possible either - unless you get a USB hub.
I'm not quite sure the purpose of your project is, but it would be infinitely easier to just buy a cheap motherboard and
cpu, and turn the 2nd case into a slave/file server and only turn it on when you need it.
The problem is, you can get external versions of each item you're looking at, connected to USB. To have these items in a seperate case, to use the other computer as a "docking station"...well...technically it would be the other way around...docking stations are the ones with no brains..I can't see the benefit/need to warrant something other then just a cheap 2nd computer.