It's good that you are planning early...especially if you are considering villa accomodations. One option to keep costs down would be to look into renting
DVC points, which would allow you to possibly keep the costs down. You could either all stay in one villa or possibly break it down into 3 studios, a 1br and 2 studios, 2br and 1 studio...whatever would be best for you. SSR resort (as opposed to the villas) might be a good option because you still get the proximity to DTD, bus stops are around the resort so not a long walk to those, and you could always put in a request for your FIL's mobility issues. Not sure if the regular resort appeals to you, but they usually have availability so if a combination of studios and/or 1br/2br rooms might be easiest to coordinate there if you were renting points.
Biggest downside to renting points I think would be that there is not much (if any) flexibilty if you need to cancel or change the reservation. If that might be an issue, then booking direct through Disney and keeping an eye out for PINs or discounts might be a better way to go. A good code with seperate rooms at a moderate might work too...but then you wouldn't have the benefit or in room laundry and kitchen.
Other things to keep in mind if you rent points is to plan as far out ahead as possible because some book up really fast at 11 months out, there is no daily housekeeping, and do your research about the resort so there are no surprises. There's tons of information out there, everything from resort photos to room layouts, so I would check those against your needs before making any decisions. DVCrequest.com has a point calculator to give you an idea of cost based on the dates you are looking at and you can compare that against direct through Disney to see if it really is a savings for you...sometimes it is not.
Another thing to keep in mind (and I only say this because it is an issue with my family), really think about whether or not you all want to stay in one room or if that will just be too much 'together' time for all of you. Sometimes it's really nice to be able to go back to your own room for some alone time...and just meet up in the mornings at one of the rooms or at the food court. This might work really well if you got one 1br and 2 studios...all meet at the 1br and then you can take advantage of the full kitchen in that room. I know in my family, with that extended time together...sometimes it's best to be able to get away from each other for a little while...keeps the peace.
Anyway...those are just some ideas and thoughts that I thought might fit your needs. Hope it helps...even if it's just some other things to think about.
I'm sure it will be a great trip!! Have fun planning!