Ask your favorite SIL to go over the guest list and estimate how many people she thinks will come. (Unfortunately, you can't do this in a vacuum). Prepare a little over half the amount of food you would need if people ate a full serving of everything. Prepare more meatballs than you think--most people do like them, also the leftovers could be eaten for dinner (not by you, of course!). And they freeze well. Cheese balls that don't get put out also would freeze well for the holidays. Instead of putting all of the food out at once, bring it out as needed.
We did a 40th anniversary for DH's parents and we're really glad we did--his dad only lived another year. And they had eloped, and never had a wedding and reception. Lots of friends and all of their relatives came. They chose not to renew their vows. This was a cake and punch reception. A few hours later, all of the relatives came back and we had a big potluck, with me providing 4 big pans of lasagna. We didn't have a party for our 40th because DS was supposed to be getting married that year..... Oh, well!