I second MSM.
On an E36, the decider is not the service costs between the 316i vs 323i, it is the CONDIITION of the actual vehicle.
One may need about R20k to fix, as an new AC system costs about R10k, and chances are most E36’s AC system’s are not blowing cold at this point in time.
If it’s an Auto, factor in a rebuild of the gearbox. This is if you plan to keep the car long term...
Other than that, the 323i might consume more fuel, and the 2 extra spark plugs may make the service factionally pricier, this is not the decider. Condition and amount of work to be done on the car is your decider!