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the cost of the dropout wont be much, you will need a diff rear wheel/hub that can take disc rotor and everything that goes along with it; hydro rear brake, cable for rear mech, cassette, rear mech etc.
pls dont go front brake only on a cargo bike, in grand scheme of things (if you source used/aliexpress) couple of hundred quid isnt a lot for safe braking.
edit: @Jonny69 already said all of this
Or perhaps there's just an easy way to disable the coaster brake from the hub and ride FBO, to use the Polo term? I am doing that anyway, pretty much.