I used a Bafang mid drive kit from Amazon, it has everything you need but the battery. Bafang is one of the best DIY kits, IMHO. I like the mid drive motor as it is better on hills than a hub drive but has a lower top end. It's more expensive too. 750W is good for us and I wouldn't go any lower. We have a 20-30 mile range, depending on how many hills there are and how hard you peddle.
Bafang Mid Drive
I used a 48V, 13ah battery as that was the best bang for the buck 2 years ago. I wouldn't go any lower than 48V and I would look at a 52V battery for a higher top end. I can go 30 mph, down hill and pedaling hard.
Battery Link
Here's a couple of forums I visited when I was doing my research:
https://endless-sphere.com/forums/
https://electricbike.com/forum/
Bruce