I just converted from a Mac Mini (2.5Ghz i5) to a Macbook Pro (2.2Ghz i7). My original thought was to do what you are suggesting. I built a rack and mounted the Mac Mini, an Airport Express and a Radial USB box. I thought I could survive without a monitor, mouse or keyboard. If I needed to address the Mini I could use an iPad running Splashtop or Dual. It never worked reliably. I ended up taking a UPS, a small HDMI monitor and a mouse to gigs. In the end, I realized that I was better off with a laptop. No worries about lose power connections, no fishing for the mouse, no monitor to carry. Initially, I though I was saving money with the Mini. In the end, I spent as much for the setup as I would have for the MBP and it is more reliable and easier to transport.
I use a Nectar Panorama 61 note controller. It is a little bit of a pain to get it functioning with MainStage, but when you do, it's so much easier to work live.
Good luck!