The HP size of the aircond will really depend on how large an area you need to cover (i.e. room size)
10-15 sqm - 0.75HP
15-19 sqm - 1.0HP
19 -25 sqm - 1.5HP
25-35 sqm - 2.0HP
Panasonic is a good brand but a little on the costly side. You can opt for Haier or Daikin or Sharp. I believe all airconds will start of being efficient at the start but will require constant maintenance to maintain its effectiveness.
If you're leaving the aircond on for long hours then an inverter would be good if budget allows. Otherwise non-inverter will suffice for short spurts.