I have a Stromer ST-1, which has regenerative braking. It's an older model, but works very well. With panniers front (via Thule rack) and back (Arkel, Ortlieb, Timbuk2 via a Topeak rack) I can carry a fair bit. I have a Burley cargo trailer for bigger shopping runs.
Most of the time, I don't need the trailer .... unless you are *always* going to be hauling more than you can fit in 4 panniers ... I think you might well be better off without a dedicated cargo bike.
I'd expect you should be able to find a used Stromer (fairly popular in europe) just invest in a new and large battery. Keep whatever it came with as a backup. I haven't seen a direct solar to battery (the factory recommended an inverter which seemed wasteful), but I'd think it shouldn't be too hard to find someone to fabricate one (just be careful not to overcharge ;>)
My cargo bike turned out to be a bit of a beast to ride.
And, since it is not electric, it stays home except when I absolutely need it, which tends to be pulling the heavy trailer.
Everything else I either use my ordinary bike, or kid's trailer.
My next build will be a dedicated tow trike for the heavy trailer.
