FWIW (probably not very much, but hey ho), each Easter I organise a get together of friends, their lovers, partners, children and misc. hangers on. This Easter we had 34 (I think). In the run up I ask for catering volunteers. One family organised the buying of breakfasts, another the makings for packed lunches, evening meals were done similarly. Cost per (adult) head was about £15.00 for the (long) weekend IIRC.
EDIT: IME, big houses such as we've used in the past, and you're looking at, will have pans of sufficient capacity to feed the number of people, plus a bit, that can fit in.