Have you considered fish as a main course? You can do cod or bass filets oven roasted with garlic, cherry tomatoes, and capers. There's a great recipe like this in Donna Hay's "Modern Classics." You can scale this to 20 people without blowing your budget. If you use cherry tomatoes on the vine, it makes for very nice presentation. You can serve it with a simple side dish like couscous with vegetables. Leg of lamb would also be an easy dish - but you may need to get 3 or 4 to cover this many people. If you want to save on the side dish prep, roast the lamb on top of thinly sliced potatoes and onions. Roasted tomatoes make an nice pairing that would add some color, too. For desert, if you want to avoid a cake, how about simple pastries like eclairs. You can bake the shells in advance and fill them the night before. Another alternative is fruit desert - like vanilla poached or baked peaches. The former should be made a day ahead and chilled. The latter you can prep ahead of time and bake while everyone is having the main course. Good luck. Cooking for a big party like this can be nerve wrecking, but it's a satisfying experience.