You might want to play around with the http://ebikes.ca/simulator to see what results you get with different motor, controller, and battery combinations, for your conditions. It'll show you what will happen with different combinations of stuff, once you get the hang of it.
If you use your existing bike to ride the way you want on that terrain, pulling a trailer with the same load weight the cart will have, while using a wattmeter to measure the power usage, that will also help you by knowing what usages you might see.
There are also other threads about heavy bikes/trikes and hills (or acceleration from stop/start, which amounts to the same thing), that will have info for you. My SB Cruiser thread, in the last several pages, has it's present configuration and load capabilities. There are also some of these:
https://endless-sphere.com/forums/search.php?keywords=cargo+trike&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search
and these
https://endless-sphere.com/forums/viewtopic.php?f=3&t=83438&hilit=coffee
https://endless-sphere.com/forums/viewtopic.php?f=3&t=88289&hilit=coffee