Mazel tov on getting engaged. 

Speaking from the chef perspective here, 1K isn't a lot of food budget for a party of 300 or so people, and you should cut back your wedding to a smaller affair attended by family and close friends. Communicate with the relatives if they aren't going to help you guys foot the bill for it, because if it's coming out of your pocket, it's your call, not theirs.

Salad is cheap -- romaine is about two bucks a head and you can probably get cases of even more cheaply. Dressing can be homemade, which also keeps it cheap. (Even ranch and caesar.) An entire case (24 heads) ought to do a party of 200.
By the same token, chicken wings are cheap and if you have access to a deep fryer, so much the better. Or you can bake them and serve them up with sauces. If you have the time, brine them overnight, then smoke them, then deep fry them. Serve with an orange sauce.
Chex mix is hella good; if you get 1/2 boxes of corn, wheat and rice chex, 2 16oz containers of peanuts and 1/2 pound bag of pretzels, you can use two salad dressing packets (like Italian or garlic and herb) and do that in stages, with 10 (2 1/2 sticks oz of butter melted and poured on the mixture of all these ingredients at 300F for forty minutes, stirring every ten. I tend to season with salt and garlic powder to get the taste up to par. This is my typical party snack mix. A large party will probably require four to six loads of this stuff.