Also arguable if not a bit pedantic: is it really DIY if you have tens of thousands of dollars worth of machine shop tools to help you do the work? It’s more just fabrication at that point.
That said, if you go back through this channel’s archives, you’ll find they actually built the majority of the tools they’re using here.
Make a spot welder for cheap!! (2012): https://www.youtube.com/watch?v=vrlvqib94xQ
Homemade Stick Welder - From Microwave Parts! (2013): https://www.youtube.com/watch?v=nTDx3sN2dhU (This one looks terrifying)
Making an ARC Welder - Part 1 of 2 (2013): https://www.youtube.com/watch?v=r6oDCbcmtWw
http://promotingprimitivism.blogspot.com/2014/01/the-workman...
I accept that this question (of whether personal skill and judgement is involved) might not really be the same as the question "is this really DIY?" but it's interesting all the same.
If you are willing to bolt everything together, you only need a drill ($40 for a corded one), small number of drill bits ($20) and a hacksaw ($20). The curved metal can be approximated with 2-3 straight segments.
Most of the expensive tools come from the choice to weld and have fancy curved metal bits. A bunch of the other tools are just faster than using a hacksaw.
A pillar drill
A MIG Welder + Mask
A chop saw + cutting disc
A pipe bender
Some spray
You must have really expensive tool shops round your way
A lathe (how many people have that one in their toolbox?) An angle grinder A metal nibbler