Choosing parts for wheel build on a cargo bike conversion
Do you have any suggestions on what type of rim, spokes, hub, rotor and brakes I should use?


For cargo bike purposes you obviously should prefer sturdiness over light weight for wheel components. 

A wider rim would allow  you put wider tires which can help with high cargo weight. For 700c wheels, rims that can accomodate tyres of at least 40 mm width are reasonable.

Previously, one would recommend going with bigger number of spokes per wheel in such wheels — 48 if you are really dedicated to find them as not many vendors offer them, 36 if possible, 32 if no other option, and avoid modern rims and hubs with less than 32 holes. However, I am unsure if this advice keeps up with the times, and it may well be that 28-hole rims are as good as 32-hole these days.

For spokes, use the thickest straight-pull available. For nipples, use brass ones.

For hubs, use compatible ones. Given that you don not plan  to spend too much money on the project, just use the best you can find to match the rims and the frame/fork.

I do not think that it makes any difference as of which rotors to use, as even the cheapest ones stop things as efficiently as the fanciest ones, while being only heavier.

For brakes, it depends on the terrain where you would ride this. If the area is mostly flat, I would say any decent modern disk calipers would do the job. If there are prolonged descents, then you'll have to look for four piston downhill-rated options for front wheel. With 203 mm rotors, look after the compatibility — most likely you will have to use adaptors from 160/180mm to 203mm.