As producer, meet with the director and director of photography to truly understand the scope of their vision and how many "specialty" shots they have in mind.
Have them bring you a list and/or storyboards of intricate shots so you know how much extra equipment to account for.
Make a list of the general equipment needed. This includes the cameras, film/tapes, lighting and sound.
Add to that list all of the luxury equipment, such as a dolly or crane, or anything needed to pull off specialty shots.
Factor in items not necessarily for filming, such as food, computer hard drives, a TV monitor for playback or even clipboards for the crew.
Using the money you have, carefully research and speak to rental houses to determine how much needs to be allocated to the essentials.
Always leave about 10 percent of your budget as a contingency fund in case of emergencies. This way you will have some "leftover" money to spend if need be.