I spoke with my vendor, he says putting a high wattage / voltage motor on a larger wheel tends to warp much easier than lower wattaged ones.

As an example, you get alot more torque out of a 20" tire than a 26" tire given the same amount of wattage.
He does have a true 750w 48v motor, but he will only put it in a 26" wheel since it won't fit on a smaller wheel nor will a larger rim like the added torque.
The price jumps up $60 for that motor, but doesn't help you because it's in a different size rim than you need.

He only deals with SLA for the local purchasers, because the cost of shipping lifepo4's is cost preventative for decent profits, and he likes to sell as cheap as possible.

I know you can get a 36v 20ah battery from eBay for $389 including airmail shipping (2 week delivery time)
I have purchased 2 of those and they've been working well within the 20ah stated. (they're running on two pedal assist only bikes) which cuts down on power requirements and c rates.
So I checked and that same seller has 48v 15a (same w/h) for the same price. OR that same seller has a 48v 1000w motor and 48v 20ah batt for $800 shipped in two weeks.
But that seller is mostly a battery seller and not a motor seller normally.
Their 48v 20ah is $529 with 2 week delivery time, add that to $214(+$10
for 700c) for the local (usa-free shipping) motor vendor and you get a total of $743. And have everything in two weeks or so and