I think your confusing AU with AAX.
AU is Apple's native plugin format for Logic, Garageband etc (although VST is also supported on OSX). AAX is the plugin standard for ProTools and is available on both OS (Windows and macOS).
And yes, you definitely need a Apple system to compile AU plugins (and AAX plugins for ProTools on macOS). A mac mini will be just fine as long as it has an Intel CPU (you can't compile Universal Binaries on PowerPC CPUs). I am myself using one as main system (I am running Bootcamp for Windows so I have both OS on the same machine).
In order to compile AAX plugins, you need to register at Avid, compile their SDK, get a iLok, register at PACE for a code-signing account, get a digital codesigning certificate for Windows from Comodo (or another company), build the AAX versions and sign them using their codesign utility which I can tell you is a very tedious and annoying procedure.
RE Commercial license, I am cooperating with a few companies that are building commercial plugins, where I offer my support during development and after release. If you want to discuss the specifics, drop me an PM.