By "affordable", I'm assuming you're talking about the price of renting a party/catering place, and not about the food because price they charge varies by what you wish to serve your guests. I've heard good things about catering companies/venues in Teku and Baneswor w/ ample parking space. One of my best friends recently spent Rs. 800/plate and the food they catered was really delicious!

If the venue that Hurray mentioned happens to be located on the opposite side of Everest Hotel (opposite side), then I can certainly vouch for it. They had great underground parking facility - both for cars and motorbikes.