A great taco party lives or dies by the food truck you choose. If you want a specific local partner for your backyard, you need to lock them in at least a month before the guests arrive.
You are looking at about $900 for a solid catering setup. Spreading that cost out keeps your bank account stable while you handle the drinks and decorations.
That 18-month window is a deadline, not free money. If you have a balance left over when the time is up, the bank will start charging interest on whatever remains, which gets expensive fast.