Approximately how many classes a year?
4-5.
Does this mean that I have to pay the costs of transport etc for the second stage interview?
No. If you did Stage 1 outside HKG, you will be invited to the company headquarters in HKG, where you will do Stage 2 (and 3 if successful in Stage 2). Cathay will pay for your plane tickets and accomodation.
Is there a minimum term of service once you finish training?
No, you are not legally bound to stay in HKG for any period of time. But where you would go with the experience of a cadet graduate? That is the main obstacle preventing you from seeking employment elsewhere.
Hope this helps!