Insulation question about air handler and crawl space

There is so much information about the subject of insulating a crawl space, i've done a lot of searching and have come up with the basic ideas of what I need to do.
I have a crawl space, with attached basement (a small 3x5 opening). The only way into the area is from a small outside door into the basement.
I've gathered that I should be insulating the walls of my crawlspace, laying down a protective barrier, and sealing everything off. I have an airhandler/ductwork inside the crawlspace and a water heater in the basement. Do I need to do anything else? should I be worried about the air handler down in the crawl space?
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