A few notes, based on my experiences with my long cargobike Crazybike2, and my SB Cruiser cargotrike:
Between 15-20MPH, air resistance starts to make a lot more difference in power usage than below that. The boxier with more stickey-outey-stuff there is, the worse that problem becomes--although the longer the bike is, the less of a problem it can be, if most of the surfaces along the sides are parallel(ish). Sometimes.
The heavier it is, the more power it will take for acceleration from a stop, and also on uphills.
Conversely, it'll have the potential for greater regenerative braking energy recovery on the downhills or slowing down from speed, if you used a DD hubmotor. (though the DD hubmotor has disadvantages over a middrive for some of your situations).
Middrive, if gearing is optimized for your speeds and loads, and you shift properly as needed, will be more efficient than the DD hubmotor, if you are at different speeds under different loads.
For my CrazyBike2, I might get 20-25wh/mile power usage, none of it from pedalling, with a 2WD hubmotor setup, cruising at 20MPH (average somewhere around 15-17MPH I'd guess), accelerating to that speed from a stop in ~4 seconds (which uses a lot of power). Lower wh/mile on longer trips with less stops than shorter trips with more of them, so a small but significant part (several wh/mile) of my power usage was accelerating the mass, rather than overcoming wind resistance at speed. Could've reduced the power usage with a fairing, even partial, like a tailbox instead of the squared off backend from the cargo boxes and seatback.
Batteries: The higher the battery capacity, for the same c-rate of cell, the lower the load on each cell, and the easier it is on them, and the less each will sag in voltage.
Generally cells that are meant for higher capacity per-cell will be less capable of supplying that energy quickly, while cells that are meant to suply that energy quickly will be less capable of holding as much energy total per cell.
Batteries can get heavy. My EIG NMC lithium packs are 20-something pounds for a 14s 20Ah pack in a steel 50cal ammocan, and under 40lbs for a 14s 40Ah pack without a case around it.
There are lighter cells/packs; but generally I'd anticipate nearly 1lb/Ah for 14s packs with a sturdy crash-resistant enclosure, and then if they're lighter, well, bonus.
48v can mean 13s or 14s, but usually it is 13s, and 52v is usually 14s. Typically doesn't matter which it is, as far as the cotnroller/motor/etc are concerned, only for the top speed of the system, for the most part. See the http://ebikes.ca/simulator
to play with different systems to see what I mean.