Have you settled on a time of year to go? If not, we loved September. We were in Hawaii for 16 days and didn't see rain once. It was also a quiet time of year with school back in session. Worked out great for us.
If you decide to go with renting DVC points, most brokers will require full payment at time of booking. There may be a broker out there that doesn't but I don't know who. If you find an owner yourself, which I would not recommend for a first time point renter, you can usually work out a deal for 50% down and 50% closer to your travel dates.
Will one of you have a license before you travel? Having a rental car will allow you to see much of the island at a fraction of the price that organized tours would cost. You'd also get free parking staying on rented DVC points. And it would allow you to stop at a grocery store and get inexpensive snacks, drinks, and stuff for meals which will help save on your food budget.