When you first start to look at hubmotors, they all look fairly similar. If you are on a tight budget (as many new readers are), then it is likely that you would be best served by a "starter" kit.
A cheap yescom direct drive rear hubmotor kit from ebay ($250?), TWO torque-arms ($50) a cheap bike ($150), and a fairly good 48V lithium battery ($700?). When you decide to upgrade some part of the ebike, because after you actually have one, you now know what you really need to best suit your particular rider profile...The bike and kit can be sold cheap, so not much loss there, maybe even break even because it's already assembled and can be test-ridden by potential buyers to show it actually works?
You will never get back any money on a battery. Everyone will assume it only has a little life left in it (which is actually a pretty safe assumption on a mystery ebike for sale). Therefore, on your first ebike, I recommend getting a decent battery that you will be keeping, 48V and 15-Ah minimum in size (20-Ah is much better). I'd also say get a hardtail-framed bicycle (used and cheap, with good brakes) and load it with a triangle-shaped battery pack, in a triangle bag.
For a lithium triangle battery, Luna and em3ev are popular right now.