Assuming in the state of Michigan, I have seen this Secretary of State web page on assembled vehicles, however, the two forms attached (BFS-72) and (TR-54) seem very basic and non-technical for the most part. My question is is it really that easy to build a vehicle that meets the criteria, if so, are there any restrictions as far as obtaining insurance for such a custom-built vehicle, and lastly would it be able to be driven out-of-state?
My question is is it really that easy to build a vehicle that meets the criteria
That's not a legal question.
are there any restrictions as far as obtaining insurance for such a custom-built vehicle,
That depends on each insurance company's coverage offerings.
and lastly would it be able to be driven out-of-state?
That depends on the laws of states other than Michigan regarding the licensing of custom built cars and the coverage of insurance companies licensed in those states.
The sections of the Michigan Vehicle Code listed at the bottom of Form TR-54 (MCL. 257.685-686, 257.688-691, 257.697-697b, 257.705-711) provide more specific information about what the police officer will be looking for when they make their safety inspection. You can read the pertinent sections of the Michigan Vehicle Code to find out more information.
Regarding the specific questions raised as a comment elsewhere, the requirements for headlamps are given in MCL 257.685, and those for brake systems are given in MCL 257.705 & 705a.