New to eBikes - where to start?

Hey everyone!

Just joined the forum and I'm super excited to dive into the eBike world. I've been riding a regular mountain bike for years on local trails (mostly singletrack, 10-20 mile rides), but I'm looking to get into an eMTB, ideally something I can build or modify myself. For those of you who started from scratch, what was your biggest "aha!" moment or the most crucial piece of advice you got regarding DIY eBike builds or conversions? I'm curious about everything from motor kits to battery choices for trail riding!