Why is caring for iguanas and having various snakes insufficient to even begin to apply toward keeping a varanid?
In general terms, I think maintaining (in decent health) an iguana is excellent experience and preparation for keeping a monitor. I have both and both are challenging in their own way. Regardless of how much we love monitors they are still lizards and not some alien lifeform with bizarre needs or care.
In fact, although I have never had an ackie I have had quite a few monitors. My feeling is that ackies might even make an excellent beginner lizard, along the lines of beardeds.
I also still believe that savannahs make decent beginner monitors. They do not get that large and are often very forgiving of mistakes (both in handling and in care). Even wild caught ones usually do well if kept at the minimum level of care. All this "you must take them to a vet because they are WC" crap is overkill. Have a basic level of knowledge and a WC one will do fine. Is it right to be taking WC ones? That is a different question than whether or not they make decent "beginner" monitors.
What exactly should constitute sufficient experience with reptiles before getting a monitor and why and how does one get that experience?
I agree there are some monitor species that are probably less rewarding than others as a first time monitor. Niles rank first among the easily accessible, unsatisfying monitors. Prasinus are also among those that are probably not very satisfying as a first time monitor, though a lot harder to acquire. Timors are in that list too.
NOTE to the guy getting a prasinus. You are choosing a very difficult monitor and you may enjoy some of the many other available monitors more than you will a prasinus. Look around before committing to a prasinus.