There are LOTS of things, besides how much food, you need to deal with. You need event insurance (business liability to cover staff and guests in case of accidents, food poisoning, etc). You need written contracts specifying lots of things, including who is responsible for setting up and taking down the seating and tables (and renting all the dishes, tables and chairs, chafers, providing linens and centerpieces, etc.) Also, a party this size, buffet service, would normally have 3 cooks and at least 8 servers.

This is a pretty big party for a first catering adventure...

You will need about 3 cooks (total) and about 6 servers per 100 guests for buffet style service. You need a written contract that specifies what food, beverages and set up, service, and equipment you are responsible for including in the price. If this is a paid event, you PROBABLY need a public-health inspected kitchen to work out of, unless there is one at the site you are using- check licensing requirements in your state. You need LIABILITY INSURANCE for the event- what if someone gets sick, claiming food poisoning, or a guest or server is burned or cut?

Here are a few rules of thumb for you:
-Food costs should be 1/4 to 1/3 of the bill, the lower fraction with higher service costs (tables with plates,) the higher amount is OK for a buffet.
-Total weight of edible food for 1 adult, 1 1/2 pounds for a meal, 1 pound for a reception type event.
-Total volume of all sides for bbq, salad, slaw, beans, is about 1 gallon for each 10 persons
-Biggest cost is the meat, so menu choice steeply affects price; for riblets or ribs, you need 1 pound raw per person, where for pulled pork sandwiches, you need about 1 1/4 pound boneless raw for 3 people. Chicken pieces or leg quarters are a big meat bargain these days- check the plan for 100 lists for amounts.
-Choice of meats increases amounts needed about 10% for each one added. For example, say you want to serve Italian beef (sliced, marinated) AND baby backs in equal amounts to 100 people; you use the charts for 100 people, but instead of making for 50% each, you make for about 60% each- people eat more and take some of each when offered a choice.
-To control costs, put all the salads and side dishes first, meat last on the line. Put desserts very visible but on a separate table so people see them and "leave room for dessert".
-To control costs, have someone serve the meats- for a large party, the cost of the server is less than the cost of the extra meat taken with self serve. Besides, it looks elegant.
-To control costs, add some side dishes such as borracho/ranch beans, an extra slaw, or a hot vegetable or rice or potato dish that goes along with the menu.
-To control costs, use a 9" plate instead of a 10" and use a slightly smaller serving spoon. People usually take 1 or two spoonfuls regardless of the size of the spoon
-To control costs, be sure that the written contract specifies exactly how many persons you are responsible to serve, who owns the leftover food, what is going to be done about no-shows (you are contracted for 300 people, 265 show up, they want to pay you for only 265. Your entire profit is gone; you worked the whole event for free...) and what your responsibility is for extra guests (you are contracted for 300, 325 show up, you run out of food. Who gets blamed?...)
-To control costs, be sure the contract specifies who owns the extra food- if you bring 10% extra above the contracted amount to cover extra persons, not an unusual thing at informal gatherings, some inexperienced employers think they get to take home ALL the food not eaten. Un-unh, see why the contract specifies exactly how many meals they are buying?

Read all my food safety articles before you start selecting the menu and planning the prep. The storage and kitchen facilities may affect the menu.

Finally, go on line and inspect prices for comparable meals in your area. You will find some consistency, and there is a reason for that. If your estimated price differs markedly, ask yourself, why? As to price, professional caterers charge 3 to 4 times the food costs, depending on the type of service, number of staff supplied, table and equipment rental, insurance (you DO have liability insurance, don't you?) and many other factors. See the pro catering board at chef2chef.com or cheftalk.com for lengthy discussions of amounts, contracts, etc.

For spaghetti quantity help, see the spaghetti dinner for 50-60 at the bottom of the spaghetti page. You would do 3-4 times the amounts, depending on whether it was really 150 or 200