New to eBikes - where to start?

Hey everyone!

Just joined the forum and super excited to dive into the eBike world. I'm completely new to this, but I'm really leaning towards building my own custom eBike rather than buying off the shelf – the idea of tailoring it exactly to my needs is super appealing. I'm thinking of converting an old hardtail mountain bike for mostly commuting (10-15 miles round trip) with some light trail riding on weekends. For those of you who've gone the custom build route, what's the absolute first step? Should I focus on the motor kit first, or is there something else I should be considering before anything else? Any advice for a newbie would be awesome!