New to eBikes - where to start?

Hey everyone! I'm completely new to the eBike world and have been doing a ton of research, but honestly, my head is spinning a bit. I'm really leaning towards building my own custom eMTB for some trail riding and longer weekend adventures (thinking 30-50 miles on mixed terrain), rather than buying off the shelf. For those of you who've gone the custom build route, what's the absolute first thing you'd recommend focusing on – the motor/battery combo, or finding the perfect donor bike frame? Any "wish I knew this sooner" tips for a newbie would be amazing!