First: Considering RENTING a lens if you are not sure you want to spend the money for it. There are lots of online lens rental stores.
Here's what I'd look for in a lens for weddings, portraits, and available light:
A FAST 85, 100, or 115mm length lens. (Maybe a zoom in this range--they can be handy if you don't want to bring an arsenal.)
Fast refers to the aperture number (lenses like f/2, f/1.8 would be fast, f/4 or greater would be slower).
A faster lens is vital if you plan on doing any available light and/or indoor work. Your images will turn out dark, blurry, or noisy (too much noise from high ISO) if you are shooting in light that is too low for your lens; and a standard built-in flash isn't very flattering--nix the on-camera flash as much as possible.
85mm and longer lenses are also much more flattering (for human subjects) than shorter lenses and you may have better depth of field options (more of the pretty background blurriness while keeping your subjects in focus) with a longer, and faster lens. This can help make beginner style photography look more professional--kind of an easy "cheat" to get good photos.