Congratulations - especially for being so smart in the way you're going about it. I'm always astonished that some people actually go into debt & have to finance something as DVC. Don't get me wrong - I love my dvc and think it's a great long-term value but I also believe in buying stuff when I have the money to pay for it in full.
Hope the 18 months fly by, and you'll have many years to enjoy your dvc without paying interest on it!
I did the same thing a few years ago. I opened a separate savings account specially for DVC. When I had enough money saved we bought OKW points, then I added to the same savings account in case we wanted to buy more points, which we did. Now I am thinking about -adding to the savings again for ... more points. Paying upfront certainly reduces the stress.
If you're buying through Disney directly you can put the entire balance on your Disney visa and then pay it off without interest in 6 months. It's like making a cash purchase except you earn Disney dollars and get your contract 6 months earlier. We always pay our contracts in cash but last time we put it on the Disney visa and liked this approach. We just kept the our cash in the bank a few more months.
I predict you'll succeed even sooner than you think. I'd be thinking of ways to cut costs all over the place. Butter for toast??? It's healthier plain, and just one slice for the 3 of us.(kidding..kind of)