What is a SEER rating?
SEER rating of an air conditioner tells how efficiently the unit uses electricity to produce cooling. The higher the SEER rating the lower your hydro bill. Old unit that we are replacing these days had SEER rating 10 or less when they were new 15-20 years ago. New air conditioners have SEER rating 13 or higher.
The cost of air conditioning for a household depends on the usage and the cost of electricity. The cost of electricity in Ontario is rising and will keep rising. Your new unit will be cooling your home (and using hydro) for the next 15-20 years. Unless you are selling the house soon, it does not make sense to limit yourself to an entry level unit.