The kits are mostly for DIY's who are going to do this once. If you think you are going to do it again, on your car, other cars, it will work out better if you buy separate supplies instead of a kit.
Bear in mind that you can use the same polishes and compounds you use on paint, as well as sealant. So the only thing you need to buy if you already have that stuff is the sandpaper, which you can get here at AG or you may be able to find a local auto body store.
If you just use a regular paint sealant, you may need to re-polish your lights every few months. For something more long-lasting, consider a coating, or a specific product like Opti-Lens, or the spar urethane method.