Based on my experiences with CrazyBike2
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=12500
(which is up to 380lbs or more with me, the bike, and middling cargo loads), you could just about do what you want with a front 9C or equivalent and a 48V20Ah LiFePO4 battery, on a 12FET 40A controller.
You'll need that 40A only for moments at a time, but you'll need it, at startup from stop, to get going with the weight on there.
I'd also recommend 24" vs 26", or maybe even 20", just because you'll get lots more torque out of it with less hardship for the motor at lower speeds, and though you have a lower top speed it doesn't matter--you're only wanting 20MPH. Based on http://ebikes.ca/simulator you could get 21MPH out of a 20" 9C 2807, using a 48V9Ah LiFe and a 40A controller, on a 400lb mountain bike. Assuming no starts and stops (yeah, I know) running at 20MPH you would get about 37Wh/mile out of that, which is a bit worse than what I get out of CrazyBIke2 on my typical heavily-loaded grocery runs, which might give you a 10-11 mile range, pulling something like 16A from the battery.
With a lot of stops and starts you would get less range, because of all the power wasted getting you moving.
Using the Ping or other reputable 48V20Ah (actual 20Ah usable capacity, not just advertised as that) you would have a lot more range. Using RC-LiPo, you'd have much better acceleration than a typical LiFePO4 pack, though a Cell_man A123 LiFePO4 pack would probably do about the same as the RC stuff (not sure which would ahve less voltage sag).
FWIW, if you want to haul the trailer, I'd use a rear motor, not a front, cuz you won't get much traction out of that front wheel, based on my experiences on my more normal DayGLo Avenger bike,
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=15570
with various trailer loads and whatnot, up to several hundred pounds of dogs or dog food or junk or whatever.
http://endless-sphere.com/forums/viewtopic.php?f=2&t=18671&start=0
Chain-drives, (BB drive, mid-drive, Cyclone, StokeMonkey, etc.) are a lot more efficient/powerful at starting you from a stop/acclerating and climbing hills, as long as you change gears properly for them to do so, or use something like the NuVinci hub developer's kit to automatically do that for you (about $160 or so shipped, IIRC).
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=30641
Buuut: chain maintenance, especially if you don't have perfect alignment, can certainly be problematic. It is the main reason why I am running a hubmotor on my current bike, CrazyBike2, when I had developed (after much trial and error) a nice powerchair-motor middrive to run thru the gears. I kept having frame twisting cause chain misalignment and total destruction of chains, sprockets, chainrings, and in one case even my entire rear wheel.
In traffic, yanking me to a sudden and complete halt.
That said, chain drive works very well when no alignment problems exist, but the chain will still wear faster than usual, as will the sprockets/chainrings, especially on a heavy bike. I have a new bike in progress that will be chain drive, with a frame designed to prevent the misalignment/twisting problems I had before, but it's not done yet, so can't tell you how successful it might be.
http://endless-sphere.com/forums/viewtopic.php?f=28&t=31255
All in all, for reliability, I'd go with the hubmotor, since where you live is as flat as it is here in Phoenix (flatter, actually). As long as you can keep saltwater out of it and the rest of the electronics, it'll work well and for a long time.
I'd also go with http://ebikes.ca, because they've got great customer service, develop neat and good and useful new stuff (like the Cycle Analyst), and their prices are good (the bit more you pay is worth it for the help you'll get if you do have problems down the line). Also because they exist to further the ebike "rEvolution", and not just to stuff money in their pockets. :lol: