Moorish Idols are absolutely impractical for any but the most advanced of fishkeepers. Even many well-respected public aquariums have difficulty keeping them alive. If you don't know if you know enough to keep one, quite honestly, you don't.
I've worked with fish for many years, although admittedly saltwater only in the two LFS's I've worked at, and when I finally set up my own saltwater tank I wouldn't even CONSIDER a Moorish Idol. (And I'm the person largely responsible for my current store having the best, healthiest, thriving sea horse tank of any store in the area.)
If you like the general look of the fish, please consider a
Heniochus diphreutes, or the "schooling bannerfish". While they are butterflies, which makes them moderately difficult (although once they settle in and accept food they usually thrive), they're sturdy as tanks compared to the actual Moorish Idol. But be SURE you get the
diphreutes, not the very similar looking
H. acuminatus.
Acuminatus is a notorious coral polyp picker, and can do some real damage in a reef tank, while
diphreutes is one of the very few generally reef-safe butterflies and tend to leave corals alone. Order the fish from a source you TRUST.
Diphreutes do also prefer to be kept in multiples, as the name implies, and a small school of 2-3 will make them happier than being singles.
Here's a good photo (not mine, just from Google) of the "schooling bannerfish".
