Can anyone help with quantities for a buffet?

[Wellmet]

I need to cater a party for about 100 people. It's a social club type affair, and we're self catering to keep the cost down.
However, I've never catered for such a large number before.

It's a friendly crowd with lots of kids, so I'm thinking of sandwiches, crisps, sausage rolls etc.

Can anyone link to a plan or anything to help me figure out what to buy?

Usually allow for like...

2/3 sausage rolls per person
2/3 small sandwiches per person

15 large bags of crisps for bowls
Pop corn is a good padder out too!

What type of party is it Op? Lunchtime with kids, early evening (people expecting a main ish type meal) or evening (more boozy but people will eat before)?

It's an evening, adult big birthday. Family and good friends. Mix of ages, from babies to elderly grandma's!