You can use that bike. But it will handle like dogshit when actually loaded up with 50 pounds of groceries. The cheap bike frames get soft as noodles when you really load them up.
But I'd rather ride that bike than any of those pre built bikes you showed earlier. To really pack freight, a longtail cargo bike is the thing you need. You might find yourself building one out of scrap bikes someday.
Here's my grocery getter, more money in the saddle bags than the bike itself. Tires and tubes cost twice what the whole bike cost. It's stiff and sturdy, made from cheap but strong steel bike frames. At least a thou in battery and motor, likely more like $1500 including the cycleanalyst.
I agree, if you go with the RC batteries, build something that will carry the batteries up front. Here is a pic of one of my past bikes, built strictly for dirt riding. I never pedaled on it, so the awkward boxes did not matter. You can carry 44v 10 ah up front easy and still have knee clearance to pedal. This box carried twice that much, and though I could pedal, it was awkward. You can carry some battery in a front bag or box on the handlebars as well.
This is the only rack you want for this bike. The axiom brand pannier rack, in the seatpost version. It is very strong. But to really handle 50 pounds, it needs bracing back to the frame. Otherwise, you just snap your seatpost. I've bent and broken several. This is what the brace looks like, just something metal attached to the frame, at the bottom of the seatpost. Drilling the frame for the bolt will be fine. Often the bolt goes through the seatpost too.
