New to eBikes - where to start?

Hey everyone!

I'm totally new to the eBike world and feeling a bit overwhelmed, but super excited to jump in. I'm really leaning towards building my own custom eBike, maybe starting with a hardtail mountain bike for some local trails and commuting (around 10-15 miles each way). For those of you who've gone the custom build route, what's the absolute first thing you'd recommend someone like me focuses on? Is it the motor, battery, or finding the right donor bike first? Any advice would be awesome!