New to eBikes - where to start?

Hey everyone!

I'm totally new to the eBike world and have been doing a ton of research, but I'm feeling a bit overwhelmed by all the options, especially with custom builds. I've got an old hardtail mountain bike frame that I'd love to electrify for some mixed-terrain commuting (about 15 miles each way, some hills). For those of you who've gone the custom build route, what's the absolute first thing you'd recommend a beginner focus on – motor type (hub vs. mid-drive), battery capacity, or something else entirely? Any advice on where to even begin piecing together a reliable system would be awesome!