I need some help and advice on heating and cooling for a 2nd story
extension in Melbourne.
The existing house is brick veneer (built in the 60) has an old gas
ducted heating (~70 vintage) 1 ~5yr old 6.5kw reverse cycle ac for the
living room at the back. There are ceiling fans in the bedroom and
living room for cooling.
surface area is ~160 sq m.

The upstair extension is ~65 sq m. 2 bedroom, 1 study and 1 retreat
area.
The existing ducted heating to the back of the house is disconnected.
(where the old reverse cycle ac is...) to reduce clearence space.
There is no room to run ducts between 1st level and 2nd level.

There are 3 ideas:
a) install an evaporative cooling with vent on upstair only. Rely on
the stair well to let the moist air to vent out down stair, rely on
the stair well to let the heated air rise up to heat upstair.
(cost ~4K)

b) install 2 2.5kw cooling reverse cyclesplit system (inverter type)
in the bedroom upstair. 1 ~5kw split system in the study to blow out
to the retreat area.
cost ~7k

c) install a ducted reverse cycle, 5pts. 7.5kw plus machine (do you
need an inverter type)
cost ~7K-10K

Ceiling fan in living room and rooms. for b) and c)

Any good suggestions? Running cost for a) is lowest but would it be
ok?
The insulation upstair is max out except the window is not double
glazed.

Any suggestion on how you would approach the problem? The pro and con
on all 3 options?