Try "Let's Go" for American English and "Get Set Go!" for British English.
DO buy the teachers guide. Although some of the lesson plans are basic, they make it easy for you to add the fun and games.
DO buy the tapes, as it's important for Ss to hear native-speaking voices other than your own. It also gives you a breather.
DO buy the workbooks, but DON'T let Ss have any units in advance otherwise they'll try to complete them before you've actually covered the material in class.
Also, make or download phonics cards [bogglesworld.com] for younger Ss and bypass recital of the alphabet until they know the sounds of consonants, vowels and diphthongs. After a month or two, they'll be able to start reading and pronouncing words that they've never even seen or heard before!