When I did a 700 sq ft addition I was lucky that my existing unit was
over-sized and I only needed to modify and extend the ducting. Get
someone to do an analysis of your situation, current and future.
If existing cant handle then you can put in a separate system or
upgrade your current system to handle the load.
If separate you can continue with oil forced hot air with AC or put in
a gas system if gas is available available. You could also do radiant
heat in the floors with separate AC system depending on budget and
preferences. Radiant can be gas or oil fired boiler.