Tink, not sure if you are checking back as we have only seen the one post from you. Hope you do, and
$8,500 is a HUGE budget for a Disney vacation. We travel with four most March breaks for roughly half of that.
Transportation. Airfare is going to hit $3,000 for 4. We drive for about $350 in gas and 2 nights hotel accommodation ($500 if we get two rooms as my kids are teens now).
Tickets can be a big cost, get the Disney 25% sale now, and focus just on Disney for now. We usually do multiple attractions, but have been using the same 10day NEWPFAM tickets for 4 years now (unfortunately can't get those anymore). Also check Undercovertourist.com as they usually have the best deal but not now that Disney has the price break - considering you are still 17 months out, buying tickets now is probably the best savings.
Stay offsite. We stay at luxury timeshare accommodations "just" offsite - 10 minutes to the parks. We get a 2BR villa with full kitchen, laundry, living and dining rooms for about $1000 for a week (all in, no resort or parking fees).
Meals. We eat light in Disney property, sticking to counter service snacks. Full meals at surrounding area restaurants are stupid cheap compared to Canadian dining. And we cook in a bit (not a lot) with simple breakfasts and dinners.
If you drive down you also get hit with $20 per day parking at Disney (or Universal), but compared to the savings it no worries.
And for those who say that you "must experience onsite at Disney to get the magic" I say, "I go twice as often as you as I can afford to, so I get MORE Disney magic than you do!"

Finally, check the Budget Board here for saving strategy. Considering you aren't going until 2018, you probably have the opportunity to take advantage of just about every tip they post there!
However you plan it out, have a MAGICAL trip!!!