Hi
It really depends on what your budget is.
essentially the equation is
the more you spend = the more economical the unit os to run = the lower the bills due to energy efficiency.
In a 3 bedroom house you can pay
approx - 2000€ for cheapo eroski/ecron type units installed
or
approx - 3000€ for mitsubishi inverter units installed (which can be up to 3 times cheaper to run)
Ive got mitsi's and generally try and get my owners to install the same or good quality inverter LG's or Similar.
Also be aware that inverter does not mean it heats and cools (most units allways do that anyway). inverter means that the motor is able to vary its speed and hence saving power.