How do I level and patch my aging, cracked and uneven garage floor?

I have heat but no insulation in my garage/shop and a floor that is cracked and uneven. I would like to add insulation (behind the existing drywall) to make it comfortable to work in during our cold northern IL winter months but think it may be unwise to put money into insulating before I get the floor fixed; if I can afford to do it.

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