I have two Miami Sun trikes.
My first is a single speed with the factory gearing. 36 tooth chain ring and I THINK an 18 tooth sprocket in back.
I pulled a 4 wheel garden/nursery cart with a 1,000 pound load capacity, for over 10,000 miles, over an 8 year period.
Yes, the trailer WAS overloaded several times, plus whatever was in the rear basket.
I never had problems with the brakes (until the straddle cable broke, at any rate, it does not have a rear brake.) or going up the "hills" (the tallest "hill" was just over 450 feet above sea level; that bridge is the highest point in the state of Florida.)
My second Miami Sun trike has the optional 7 speed (14-28 in back) which includes a rear disk brake. The front brake is a "V" brake.
Since the hills are taller, steeper, and longer in Idaho, I swapped out the 36 tooth front chainring for a 28 tooth.
The garden/nursery cart I pull with this trike has a 1,400 pound load capacity.
I've also added a Wald 157 front basket (same size as the rear basket).
To date, I have not had any problems stopping or going up (or down) the hills.
The easiest way to add a rear brake is to get a coaster brake trike hub (it has a sprocket welded to the hub body) in either single speed, or an internal gear Sturmey Archer 3 speed, or 5 speed, and replace the freewheel adaptor on the axle to a fixed gear adaptor.
If you go with a garden/nursery cart, you'll need a convertible handle (they have them on the "Big River" site). For the hitch plate, a piece of 1/2 inch plywood that extends 3 or 4 inches past the basket works great. Use the basket mounting bolts to hold the plywood in place.
I use a 1/2 inch diameter hole for the hitch pin.