New to eBikes - where to start?

Hey everyone! I'm totally new to the eBike world, but I've been lurking for a bit and am really drawn to the idea of building my own rather than buying pre-built. My main goal is a commuter/light trail bike for about 15-20 miles round trip daily, mostly paved but with some gravel paths. For those of you who've gone the custom build route, what's the absolute first thing I should be looking into – motor type, battery size, or maybe even starting with a specific donor bike frame? Any advice on where to even begin this journey would be awesome!