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 eyeing a custom build for a while now, thinking about converting my old mountain bike (a hardtail 29er) for some weekend trail rides and maybe a longer commute. For someone completely new to custom eBike builds, where's the best place to even begin? Should I focus on motor type first, or battery capacity, or something else entirely for a good all-around setup? Any advice on common beginner mistakes would be awesome too!