@ajroach42 As a hardware and embedded software designer, who makes a living doing these things for my own business as well as contract/consult for other companies and startups, and as a community leader in several popular open source projects over the last 30+ years:
you need to make a (business) decision as to whether or not you want to be the distributor for this thing or not.
the benefit is that you can ensure everyone has the same experience (you assemble the hardware, custom case, custom software load, guaranteed out of the box experience, etc.). this probably means in-house developing a custom kodi skin and plugins to achieve your goals.
the downside is that you probably will want to, up front, purchase significant quantity of your desired HW, and continuously evaluate new off-the-shelf components. so you need the capital, and you need a long-term business plan. one option here to avoid massive initial capital outlay is a custom board, which isn't impossible, but at least ensures that a single board computer going EOL won't force your hand to start from scratch again.
if you choose not to be the sole distributor, you really need to decide to support a variety of hardware solutions, and have a community help you construct and support the experience, even if that's largely just building skins and writing plugins for kodi or whatever, and porting to new hardware platforms. i know you want this to be turnkey, but if the effort exceeds what your team can do, it'll flounder for lack of support. you need that larger team (like kodi or jelly fin has) to do the work where you can't or don't have time to do.
the usual question i will ask when brought in to consult on this is: what do you want your core competency and business to be? is it producing content? is it making set top boxes? is it writing software?
the answer to that question will i think drive the answer to the "what" that you do, and the "how" will naturally be answered by that.