[Soli]

Since the OP is building a custom HTPC, it can also be argued that Win10 is for all intents purposes also a resource OS, and besides it might also be used for gaming which means a Nvidia card is likely to be installed anyways.

For gaming I don't neccesarily need Win10, I was also thinking about SteamOS somehow as dual boot next to the OS Kodi will run on. But gaming is low priority on the HTPC and will likely not be used or even installed in the first few months. Installing a Nvidia card was always one of my expectation so that is calculated in my budget.  

 To be perfectly honest. The new Apple TV 4K does it all if you can live without addons and Atmos sound (at least for the time being) . It supports auto refreshrate and auto HDR/SD switching for all native app, and I also think MrMC (forked Kodi without most of the addons) supports this?