I think 2 room villa would be big enough. As long as your friends are aware that they will have a room with a queen bed and a pull out couch for their kids (I think SSR has 2 queens in the 2nd bedroom - right?). If the 18 year old comes, she could have the option of sleeping out in the living room, or maybe 1 of the little ones can sleep in the queen with the parents, and the older can sleep in the pull out with the other little one. My DS fits comfy in a queen with me and DH, and we are not small people.
I think the BWV would be my favorite place to stay with a group. It has very easy access to Epcot and HS (via boat), and bus to AK and MK. But be aware that BWV only has 1 bed and 1 pull out couch in the 2nd bedroom (there is also a pull out couch in the living room).
You really can’t go wrong though…have fun planning!