First I would like to offer some thoughts. After considering them, if you can post as much detailed information as possible, we might be able to help more.
In the attached PDF, I just see a single image of part of what I presume you want to build. Not sure how closely you want to copy it, but for this post I'll assume you want an identical copy as you don't state otherwise.
To build it you'll need a design plan, and if you are copying an existing design you'll need much more detail and information about it than that image can tell you. You'll need to know how the frame is designed, and the panels made, and how it is assembled. You'll need the steering design, suspension (if any) design, drivetrain design, etc.
At a guess, to completely duplicate that design it's possible it could cost somewhere close to what they charge for it (or even more), between making the molds for all the parts and the materials to cast them and the waste from failed casts (or molds), and whatever the electrics cost (which will depend on exactly what you need it to do for you under what riding conditions for how long per charge). If you don't have design details about it, then trial and error development costs may inflate it well beyond their price for one.
If you don't want molded shapes for body panels and are willing to make them out of sheet materials instead, for a blockier / possibly cruder look, it could be far cheaper and simpler to build (but you will still need to mechanically design the entire thing first, if you don't have the detailed info to copy it from).
For the electrics...you should define the exact job you need it to do for you, estimate the worst case weight it must carry at what speed and the distance it must go, and the driving style you will use under what specific traffic, weather/wind, terrain, and road conditions.
This will let you use simulators and calculators like the ones at ebikes.ca to guesstimate the power and capacity requirements for various types of systems you might use to power it.
Before building it you may wish to look into the legality of the specific type of vehicle; in many places bicycle-like things with more than three wheels are no longer bicycles, and require either special licensing or are actually illegal on the roads. It would be a shame to spend time and money on something that cannot be used.
This also includes the power drivetrain, in that there may be power and/or speed limits or other specific limitations to the system you may have to constrain it to. If there isn't enough power in those limits to do what you want, the vehicle will not do the job you need it to.
If it turns out you can't build it as a quad, you might be able to build a trike (that is what I had to do here in Phoenix AZ, because a quad would be illegal but a trike is not). If there is insufficient power to do the job, you might have to lessen the job requirements; if that isn't possible you may not be able to use this type of vehicle to do it.
This is what I ended up building, that has served me for several years now:
This thing replaces Delta Tipper...er, Tripper v1.0, whcih was cannibalized to build it. https://endless-sphere.com/forums/viewtopic.php?f=2&t=22720 It's mostly Dogman's fault ;) ...er, inspiration, as he suggested it as a joint build for his trip out here this year. Tiny and Yogi discuss...
endless-sphere.com
