I wanted to build a small, easy-to-ride, easy (ish) to build cargo bike and here it is.
I have all the extra info in the description here. Hopefully, there is everything you need to build your own.

TOOL LIST

Must have tools,
-A welder
-Hacksaw
-Hammer
-File set
-Ruler and straight edge
-Electric drill and bits
-Centre punch
-All basic bike tools, Allen keys, cable cutters, BB tools, etc.

Well you could probably build the bike with just that, here are some more tools that will make life a lot easier,

-Angle grinder with a zip disc and flap disc
-Spirit level
-C clamps of various sizes
-Heavy threaded rod with some fitting nuts and washers
-Various adjustable spanners
-Drill press

You will also need all the safety equipment to go along, including a welding mask, gloves, eye protection etc, don't skimp on those.

I built this with only a very basic welder and toolset, you could weld it with pretty much anything, use what you have access to and what you are comfortable with. The bike could probably be brazed as well, it would be a somewhat different build possess but if that's what you are used to, you can sort it out.

Community bike shops are often a very good source of parts, both frames and forks, and random components. Make friends with the people there and they will usually be happy to help get your project going.