All A/C launched date is based on real life, with 18 months plus/minus, so SD should be 18 months?
You can expect A320-100 to be launched between 1980-1983, they won't be delivered until 1986+, so it's safe to keep using DC-9 until then. In fact, IMO, you can fly DC-9 profitably till end 80s/early 90s
DC-9-30 consumes 1.4x fuel but carry only 0.66x pax as compare to A320-100. But if you're flying thin and short routes, they won't be much inferior than A320-100, except costly maintenance with aging fleet. The higher leasing cost of A320 might even > extra fuel and maintenance cost of DC-9. But if you're looking for competition with the big boys, they have enough cash to prepaid 100% in advance at least 100 units of A320-100