Honestly, if your chain isn't stretched, (use calipers to measure space in links,) you can get away with replacing just the cassette or individual gears. Just recently I overhauled my drive train. I only tend to ride using the smallest gear, (highest speed,) and just that one gear was worn down horribly after 3900 miles. I replaced just that gear in the cassette, and the chain, and retained the rest of the gears because they weren't really worn.