the unit. I have also seen leases where the if the repair cost exceeds 50% of the replacement cost then the landlord pays to replace the unit, but he also gets to pro-rate the cost over 12 years and add it to your monthly rent.

Regardless I would try to get the landlord to warranty the units for the first year if you can. We are at the end of summer already and depending where you are you might be nearing the end of air conditioning season. You don't know how well the previous tenant maintained the unit, it is at least worth asking the landlord to warranty it for some amount of time.