Fat Tire bikes are pretty cool.
I like 3-4" wide tires front and rear myself.
Do you plan to ride it mostly on sand?
Sand is a tough gig, you need the power to get up on top of it.
500W isn't bad but did you see how much he has to pedal in the soft sand?
As far as road riding that short fat tire won't be any fun, and 20mph max?
A bit sissy to me.
Do you want to go electric or is gas an option?
For something at a reasonable price that could literally run circles around that in deep dry sand and cruise nice on the road at 35+ mph, this is a pretty cool design.
Sun Crusher with 4" wide road/sand tires ~$350, 66cc 2-stroke and shift kit ~$380 and your set.
I beefed that one up with some performance mods at would fly over anything, a real joy to ride.