Can my prior landlord sue us if the rental was unsafe and unusable?

A lease is a contract whereby you agree to pay a fixed amount of money fo ruse of the property over a period of time (month to month or a fixed term of years).  In this case, the landlord surely has a duty to maintain the items that you mentioned, which are likely specified in the lease.  If the landlord has not - as you said- fixed these things and they did not operate as you agreed when you took possession, you may have a good basis to offset the rent owed based on these defects.  I suggest that you not respond to the landlord.  if he sues you, then make sure that you use these items to defend and offset what is back rent owed.