Stage 1 is software only. It just makes the most of the what installed in the car already. This is where you will find the bulk of your gains. It should cost up to around $1200
Part of the reason the 150kw motor has some decent gains, is that is detuned for low emissions and better economy from the factory. (Assuming you have the Bluemotion variant)
Stage 2 generally includes some hardware changes, such as a high flow down pipe and a bigger free flowing exhaust. Also the removal of the DPF to allow the beast to breath. While this will give you a better spool response and some more power over a stage 1, the difference will not be nearly as big and the cost will be a lot higher. Also with the removal of the DPF you will start to see some smoke under acceleration. Also with the vehicle being a 2011 you could get into a bit of trouble for removing parts of the emission hardware.
Yes the gearbox should not have any issues with the extra torque.