Carol, see the discussion in the adjoining thread "Garden wedding reception" about this size party in a yard; it is WAY to big a party for a wedding on a budget, or for a group not used to cooking for parties this size.
I would use roasted or steamed mixed veggies for the simplest tasty addition to this menu. For quantities for the menu, see the spaghetti page, dinner for 50 at the bottom of the page and multiply times 8.
The big pots link is over in the left hand column, it is called "Get out the big pots".
There are a lot of expenses to include in the cost estimate, such as rental or purchase of plates, flatware and paper goods, tables, serving dishes, etc. $12-15 per person, not counting the beverages and the cake, is a starting place.