Well, you quoted it 3 posts above!!
There's NAS, wi-fi, no wi-fi, it's audio only, audio+video, no room for anything, but now there's apparently already a monitor in the garage despite there being no room for anything. Would have been helpful to know that sooner. It's a very disjointed discussion, for me at least -- maybe others can follow it, but I can't.
If you have a monitor already in there, I would do this:
1. Connect rPi to monitor via HDMI
2. Connect rPi 3.5mm audio out to cheap amp (hide cheap amp in ceiling with speakers if you want). There is no real benefit to dedicated rPi DAC with ceiling speakers -- you won't hear a difference.
3. Use wireless keyboard/mouse (like $20 Logitech K400) - just plug dongle into rPi USB so you can type and control mouse from anywhere
The rPi can also be hidden with above. If you can control rPi with an phone/tablet app, you can still do that with the above. Eliminates need to buy rPi DAC and rPi AMP.