I spent some time writing yesterday, and then we entered a no signal area and I lost it (I am on the ship).Surprised others have not joined this discussion.
Zago, the only way to start spearfishing is to actually start spearfishing, man. Mask and snorkel and cheapest gun and off you go. You can only improve from there. Which gun to buy is also not very relevant, because you will most likely end up owning many of them. If you stick with the sport, you will have buckets full of masks and multiple pairs of fins. So, which particular brand to buy is not very important. I would suggest to start with 75 cm speargun because it is easier to handle in the water than 1m gun. And it will be plenty of gun for the size of fish you mentioned.
You will also need a fish stringer or a catch bag and some inexpensive spearfishing knife, the kind you can strap to your leg or forearm. Also, do not neglect gloves.