Hi gr8tpanther and welcome to the DISboards!
Given that you'll have your own car, I think that staying offsite will be cheaper for you than an onsite room. However, it may not be as convenient and it will not have the added advantages of Extra Magic Hours or e-ride nights (if there are any in September). But, you'll get a comparable room for much less. It's really a matter of preference. There are those of us who prefer being onsite and are willing to sacrifice in other areas of the budget to make it happen. Then there are some of us who prefer the value of an offsite room. A great way to compromise would be to split your stay between the two, especially if you plan to visit Universal or any of the other attractions that Orlando has to offer.
There are package deals for both Disney and outside resorts. Most times, they are not as cost-effective as booking everything yourself. Once in a while a package deal like this summer's "7 for 5" promotion will come out. Those packages can be a better deal than booking all the parts separately. You really need to cost everything out yourself.
So far, I've been very happy with
Dreams Unlimited, as a
travel agent. They've been able to get me an AP discount on my room and helped make last-minute changes that could have meant hours on the phone (all long-distance). Getting a quote from them is easy and costs nothing. If you're considering using a travel agent, I would skip AAA or any of the online discounters like Expedia and go with DU.
As far as cheap eats are concerned, you may want to get a room with a small kitchenette in it. You would be able to fix a decent breakfast for your family before departing for the parks and you could even do some light cooking for evening meals, as well. Disney has "Home Away from Home" resorts that include kitchens in them but they are more pricey than the offsite resorts.
When eating in the parks, counterservice meals will be less expensive than sitdown restaurants (cheaper entrées, no tips). Carry your own water bottles and ask for ice water to refill them when you eat. Bring along some Crystal Light or a packet of Kool-Aid to flavor it up if water is too boring for you. Tuck a few snack items into your fannypack for when you get the munchies. A 50¢ packet of crackers can be every bit as good as a $2.50 box of popcorn. Read the
Everything You Wanted to Know About Connections Meal Vouchers Thread
at the top of this board to see if using vouchers might be a good way for your family to save money on dining. You would not be able to purchase vouchers now for 9/05 but it might be worth considering.
If you decide to book you hotel and passes separately, consider purchasing from an online ticket broker like
Ticketmania.com or
Paramount Travel . Their prices on PHPs will be much lower than Disney's. Or, if you're willing to join a buying club to get the discounts,
Hotwire Savings Club has the best prices of any of them for PHPs.
Okay, this post has gotten to be too long already. Have fun planning your trip. With over a year to go, you'll be able to plan, revise and refine it many times.