New to eBikes - where to start?

Hey everyone!

I'm super excited to finally dive into the eBike world, but honestly, I'm feeling a bit overwhelmed by all the options. I'm looking to convert my old mountain bike (a 2018 Specialized Rockhopper) into an eMTB for some trail riding and longer commutes (thinking 15-20 miles each way, mixed terrain). For those of you who've gone the DIY route, what was the absolute first thing you considered or bought? Any specific kit recommendations or pitfalls to avoid for a newbie?