The 2015 version of the ipac2 now has the interface for spinner and trackball built in. It is via additional 2.54mm pins so you don't use up your digital inputs to also connect it. I just wired up one with an old golden tee trackball and it works great.
You are good on the amount of inputs. You can also use the ipac2 shift key feature to move the admin functions to button combos rather than dedicated keys. This saves buttons, wiring, and inputs. I believe it is also more user friendly. You are certainly not near as heavy on admin keys as some, but it is something to keep in mind.