The battery you want to buy from a reputable dealer.
When I was digging into it e3ev seemed like a reputable dealer
https://em3ev.com/
The other one that was well recommended was Grin out of Vancouver
https://www.ebikes.ca/getting-started/start-here.html
The Grin website is a good staring point for figuring out what you want.
I had planned to buy a bafeng geared hub motor and controller off of AliExpress and then the battery from em3ev. Was going to be about 1100-1200. But then they put the Rad bikes on clearance for 1400 so I picked up one of those instead.
I do sort of wish I had built my own a bit as I could have gotten 54 volt instead of 48 and would have been on a nicer bike.
Your first decision to make is front hub vs rear hub va mid drive.