For that number, I'd consider buying it all from a supermarket food to order service, collecting on the day, and then just laying out. It's going to be really, really hard to cook/prepare and refrigerate for that many people. Alternatively getting a big order from a pizza place (approx half a large per person, plus garlic bread) or from a chip shop for delivery would probably work out as cost effective as making it all yourself.
Another way of doing it (assuming you are in a village hall etc and have table/seating space), is to set up 12 tables of 8, and on each of the 12 tables put a picnic basket (or a decorated cardboard box) and into each one put a rustic picnic selection for 8 - so a couple of baguettes, a couple of cheeses, a pack of cherry toms, a pack of salad leaves, a pack or two of cold meat, a few packs of sharing crisps, a jar of olives, a couple of packs of strawbs/rasps etc, a bottle of red, a bottle of white, a couple of bottles of fancy non alcoholic drink etc, etc. Then 8 plates, knives forks spoons etc and everyone serves themselves. The benefit of this is little/no prep, and it's easier to judge the amount per table of 8 and then just replicate it!
100 is a lot to cater for.