Tim, one thing you need to know is that a full tray is only 20-24 servings. Also, a 6 PM party, people will not eat dinner beforehand, so you want a full meal's worth of food per person.
Meatballs- 3 per person with rolls and cheese- plenty
Baked ziti- 2 full trays- go 3
Roast beef (30 lb) sandwiches with rolls and cheese- 25 plenty. You could definitely reduce costs by doing 12 beef and 7 each turkey and some kind of salty (ham, salami, etc). Since you may have both non-carb eaters and some non-meat eaters in your group, be SURE that at least the sandwich meats, cheeses and rolls are offered do-it-yourself style, not preassembled. You can use the sandwich planning page for amounts- since you have the meatballs, do 2/3 the amounts for 100
Pasta Salad- 2 full trays- enough, with the ziti. You can do this yourself, using 3/4 of one of my recipes for 100, a couple days ahead and save quite a bit of money.
Caesar Salad- 2 Full trays- probably a bit short; you can do this yourself, using 3/4 of one of the amount for 100 on the plan for 100 page, using pre-washed greens and bottled dressing and save quite a bit of money.
Relish tray- suggest you do the classic relish tray for 60 on my veggie tray page. Again, simple and quite a savings over store made.
Holiday party, I would add a fruit tray, and this could include dried fruits such as apricots and dates. Remember that lots of small sweet bites (4) is everyone's #1 favorite for holiday desserts.
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