Renter's Insurance/Homeowner's Insurance?

You should have unitowners insurance to cover: contents, improvements and liability. These are not expensive policies, and the coverage is generally broad with few exclusions. Since the co-op's policy covers the unit "as built" the burden of your coverage is only on the above items.

You can get very good coverage for a few hundred dollars per year - mine has climbed a bit, but it is still only about $500 per year, including loss of use and loss assessment amendments.

The co-op most likely has its own requirements for what insurance you have to have, and for how much. The co-op is an "additional insured".

insurance is an absolute neccessity.