Buy a good coleslaw and add some finely cubed apple plus a large handful of raisins. It's lovely! I make my own coleslaw, but there's loads of really good ones in the supermarkets, to save time.
Mini prawn cocktails, made in wine glasses or small bowls: Shred lettuce and put in glasses. Put 2tbs mayonnaise, 2tbs tomato ketchup, 2tbs lemon juice in a bowl. Add 1tsp Worcester sauce and half tsp horseradish sauce. Mix well. Add 200/250g thawed prawns, cut in half. Stir them in and spoon into the glasses. Put a slice of lemon on the rim for decoration. Easy and very tasty, I eat it a lot!
The supermarkets also do large bags with a variety of crisps and those cheesy puff type things. There will probably be some of their own brand, which will be cheaper and just as good.
Buy a few scotch eggs and quarter. You can often buy a variety of mini Indian snacks: bhaji's, samosas, etc and they are often sold cheaper if you buy a few in one go.
I buy a tin of bean salad in vinaigrette quite often. It's very nice and you just have to pour it into a bowl. You can often buy 2/3 ready made garlic baguettes in a bag. You just have to heat them in the oven for a few minutes and cut them up. You could buy frozen vol-au-vent cases, which are just cooked on a baking sheet. Buy a tin of chicken in white sauce, pour into a bowl and stir a bit to break the chicken up, then fill the vol-au-vent cases with it.
It's ages since I did a buffet and I'm having to wrack my brains. Hope it all goes well for you!