I was last in Cuba in April 2015 and cash was king, very few in our group managed to successfully use a Aussie issued atm card, so best to have enough cash as a back-up (there are no fees for exchanging CAD, Euro or GBPs).
If you have pre-paid your flights, accommodation (+1 for casa particulares) you dont actually need a lot of money, food, alcohol and activities are very inexpensive. More tourists visit Cuba each year from Canada than anywhere else, so I found exchange rate on CADs the best.
For flights if coming via the USA, I recommend SYD - DFW - CUN (Qantas/AA) then book CUN-HAV with Air Cubana direct (cheapest flights and generous luggage allowance) - its ticketed as a 90 mins but you'll only be int he air 60 mins. Spend a few days in Playa Del Carmen in mexico before/after. No advantage to going via LAX. Or fly via Canada - there are multiple flight from Toronto every day.
If you fly Air Cubana from CUN you can buy your $20USD visitor card (like a visa) at the airport counter cheaply, purchasing in Aus before you leave if 5 times the price. Or if flying a Canadian airline they just hand them out on the flight (included in airfare).
We had internet at Hotel De National in Havana, but none elsewhere (stayed in casa particulares in Cienfuegos, Trinidad, Vinales), and it was refreshing to go 8 days or so off the net.
Although they do not always check, a condition of entry as a tourist to show proof of travel insurance which covers medical evacuation (and does not specifically exclude Cuba). If relying on credit card insurance you will need to get them to issue you a letter/policy to prove this, if you purchase travel insurance the policy itself will state the coverage and countries covered. A couple on my flight got separate at immigration and they almost didnt let the guy in as his wife was the one holding their policy and had got through unasked.