notthe1freeman

Older MTBs were made for this duty IMO. Here's mine...can't share it enough just did about 50lbs of groceries lol

hotbike

Handlebar Clamps

Bending Coroplast

Pannier Tailbox

Big Corrugated Plastic Boxes can be made to any size to fit any bike.

Plan of the new Model 791R Tailbox

The Model 791R Tailbox solved the dilemma of whether to put Panniers on the rack, or put a box on top of the rack. It’s 21” deep, 14” above the rack, and 7” below on either side.

sebcast

I also use a milk crate on the rear rack which is permanently attached there with bungee cords. As for bike, I have a 1985 Miyata ninety.
I recently got my hands on a trailer for $20 and I built an outer shell for it out of oil cloth so I could haul more groceries. I also upgraded my cassette to fit smaller gears because of the additional weight and a huge hill just on the way to the supermarket.

sebcast

oh, forgot to mention, if you're into crafts, look on youtube for "Sewing A Bike Pannier - Kraftmade" for instructions on doing your own panniers with oil cloth.

hotbike

Motor and gearbox de-coupled. Bike is now a pedal bike without the weight or drag of the motor

Dropped the motor gearbox on the floor. Save the bolts, you need the gearbox, it has the bottom bracket.

After the blinkers were installed

First test ride

Great news! These e-bikes often have a longer wheelbase than most bikes, so when you install a rear cargo box, it is almost like having a longtail! Wheelbase is 45” or 1143mm.

brianinc-ville

Given your requirements, almost anything would do the job. But here are my thoughts:

• An English 3-speed would probably be just great. The handlebars are comfy, they come with fenders, and the Sturmey Archer 3-speed is hard to beat. The only downside: steel rims in the rain.
  
• I'm going to say no to all the people recommending mountain bikes; between the flat handlebars, the low gearing, the lack of fenders, and the geometry, they really just aren't made for city use. Some of those factors could be retrofitted, but why bother, when there are better options?
  
• Instead, if you don't get an English 3-speed and you don't mind a derailleur, I'd recommend either an '80s Japanese sport-touring bike (with upright handlebars of your choice) or an '80s/'90s hybrid bike. Or one of the modern city bikes, like the Linus, Public, or Priority.
  
• Whatever bike you use, I totally recommend Wald folding baskets. They're made to hold a paper grocery bag; the reusable bags you can get at any supermarket will also fit them perfectly. Both are way easier to deal with in a supermarket than panniers are. And you can throw any old thing in there. You can also bungee stuff on top of them, if necessary. Get the silver-galvanized baskets, not the black or white painted ones (which rust).