Shakey, I spent many years helping boat manufacturers meet flotation requirements and testing boats for flotation. Also, the rules are the same here in the USA as in Australia. So just translate pounds to kilos and cubic feet to cubic meters and you will be ok
The type of boat you are dealing with is required to float when swamped, in an upright attitude, and not lean more than 30 degrees either way, and not have the bow or stern more than a foot under water (one end out of the water the other underwater)
flotation needs are:
flotation to float the weight of the boat.
flotation to float the engine.
flotation to flotation the passengers and their gear.
Each of these requires a separate calculation.
Another thing to remember is that most things don't weight the same underwater as they do in the air. Some have positive buoyancy such as wood, and some very negative such as engine blocks. So the amount of flotation has to be adjusted for each material based on it's weight underwater.
Recommended foam density is 2 lb. i.e. 1 cubic foot weighs 2 lbs and supports 62 lbs in salt water.
One of the hardest parts of this is to get the engine to float and do it without rolling the boat over. This means placing foam blocks at each rear corner in amounts enough to keep the engine afloat and stable.
The foam has to be distributed around the boat to float it level and the flotation to float the people has to be divided and distributed on each side of the boat. If you put all of it in the bottom of the boat, the boat will float level, UPSIDE DOWN.
For more info look here. As I said the rules are the same here as there, http://newboatbuilders/pages/flot.html
click on level flotation.
As for pour foam vs block (sheet) foam. Whatever you use make sure it is closed cell foam.
Block foam advantages: Convenience, availability. Usually polyurethane, not affected by gas, oils, cleaners
Disadvantages: requires strict attention to manufacturers instructions. Sensitive to temperature and humidity while being made.
Notorious for absorbing water even though it is not supposed to.
Usually 2 lb density but may vary if mixed wrong.
Block foam: Advantages: also convenient, readily available from hardware and home improvement stores (I don't know if you have Home Depot or places like it). Any place that sells materials for home building sells sheet foam. Ask for insulation foam. Usually it is in 2 X 8 ft sheets, 2 inches thick, I've also seen 4 inch thick. It is 2 lb density: i.e. one cubic foot weighs 2 lbs. Does not absorb water.
Disadvantages, has to be cut to size.
It is almost always styrofoam (polystyrene) which is dissolved by gas and oil and some cleaners. So it has to be up out of the bilge, or sealed up in epoxy or plastic bags. If you use resin it must be epoxy. Polyester resins will turn styrofoam into goo.
None of this is rocket science. Anyone can do it. it doesn't require a lot math.