My choice would be a good flashlight, then night sights, and last would be a laser system.
You don't necessarily need a handgun mounted light, and there are some schools of thought that say if you are only going to have one light that you don't want it to be weapon mounted. This is because you don't want to point a gun at everything that you might need to illuminate. Weapon-mounted lights (WML) also makes it hard to use intermittent and indirect lighting techniques while searching.
My suggestion would be to get a good hand held flashlight first, then tritium nights sights, then a WML, and then a laser if you want one. Of course a combo WML & laser such as the Streamlight TLR-2 would be an option as well.
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