Lens for low-light concert photos?

I'm shooting with a Nikon D3000 and I have to use my regular zoom lens it came with for concerts because the 50mm f/18 I have is not auto-focus so it is hard to focus on things in such a dark setting. So I was interested in getting the 50mm f/1.4 AF-S. Is this a good option? And how well will it work with things in low- lighting and motion? I have a lower budget so this is one is right in my price range as well.

First, you don't have to auto focus. Before auto focus we had to manual focus.
Second, try bumping the speed up and do some experiments with low light and faster speed like ISO 800.
The biggest trouble you are going to have is working with the d3000 which is not such a good choice for low light situations.
Instead of buying a new lens I would sell the d3000 and go to audorama and buy a refurbished d5100 for a little more than $400.00
It has an excellent sensor and is much better for low light situations and comes with a nice VR lens to boot.

I use a 24-70 mm f/2.8 lens when I have an on stag media pass

Otherwise I use the 70-200 mm f/2.8 when shooting from the photographers well.

Edit:

The stage lights are not really low light. If you look, there are pools of light on the stage set up by the stage director. If you meter on those pools of light, you can shoot at higher shutter speeds. Just wait until the performer is in the brightest lights before you press the shutter release.

Remember to set your with balance to cameras white balance to incandescent so that you get the correct colours in your shots. If you can, shoot in RAW so you will have the best possible images.