Definitely know the feeling worring about the cost/decisions when looking at a big invitation list! We are now working on the 2nd DFTW for our family (my cousin just got married in Nov at WP/GF). For us, we have found that the 70-80% acceptance rate that Disney states on how many of those invited that will actually come, is a bit off.
We are expecting approx 125-150 guests, for our wedding next Feb. However the invitation list is considerably larger than that. We just started gettng our preliminary RSVP's back a week or two ago and thus far we have 40-50% yes'. My cousin's Nov DFTW final count was 33% yes.
For the decisions, as others have stated, unless you are getting the new David Tutera Collection, you will have a say in all the decisions that are made. Yes there are some stipulations, like no outside floral/entertainers for in park weddings, but generally we have found the DFTW people to be very flexible and accomodating. As to items that you will have to sort out yourself (DFTW has reccommendations on vendors but doesn't personally take care of these things) theres: the officiant, your dress/attire, BM's dresses/attire, guy's tuxes, the rings, cosmetologist/hairdresser, and the invitations.
There are definitely several ways to cut down on costs, like using outside photography/videography/floral, shopping around for your invitations (we've found that there's as much as a 35% price difference for different vendors for the same Disney invites!), making/buying your own favors, etc. There are many brides on this board who have had some excellent tips on this topic, so make sure to ask for help if you need it!
HTH!
Brandi