I'm probably waaaaay too late in answering to help you out with your decision (which I'm sure you've made already) but maybe my answer will help others who have a similar question. My own situation is similar. I've recently made some connections in my local music scene which has resulted in me shooting some small venue gigs where I am allowed to get up close to the stage and the performers (including DJs, like you) and shoot from a variety of positions (as opposed to shooting from the pit at an arena show for 3 songs, with no flash). I occasionally use my SB-700 speedlight (and am allowed to use a flash), but prefer not to overdo it. When I do, I use a diffuser and/or bounce off the ceiling. The light in these clubs is often VERY low and the majority of the stage lights used seem to be orange or red and so are really tricky to work with. Get used to correcting white balance in post.
I have a D7000 and a recently acquired D600 and shoot mostly with the full frame D600 but will still use the D7000 as a back-up. It's important to take into account the reduced field of view of the APS-C sensor on the D7000 because it makes a huge difference to your choice of lens. Be careful reading other's recommendations because they may be using the lens they recommend on a full frame camera. The low light sensitivity of both these cameras is fantastic and I often have to push the ISO up to 3200 or even 6400 with a pro 2.8 zoom (I own the holy trinity of 14-24mm, 24-70mm, and 70-200mm) in order to shoot at a shutter speed of 1/125 or 1/160 so that I can freeze action somewhat. In venues like these, faster shutter speeds result in really dark photos, but going much slower than 1/125 runs you the risk of less than sharp shots, especially if you're being jostled in the crowd. In clubs like this I'm finding that you have to be quick and be aware of your surroundings and rarely have the opportunity to rest your camera on something to steady your shot. So you're probably going to be shooting faster than 1/100th.
So, after all that intro, here's what I'm finding and what I recommend:
Make sure you are shooting in RAW and are using Lightroom for post (I have no experience with Apple's Aperture but I'm sure it's great too). This will save a number of your shots (both in correcting exposure levels and white balance and for doing some noise reduction on those high-ISO shots, of which you'll have plenty if you're using a 2.8 or slower lens). Some of the best shots I've taken have been with my cheapy 50mm f/1.8g (and you've already got a fast 50mm) and my 85mm f/1.4D, both wide open. BUT those shots were crops (in camera) of, say, a guitarist or singer or drummer and have a very shallow DOF. In other words, they're great for shooting a single performer and getting their expressions and part of their instruments, but will not get other performers on the stage beside them or in the background (due to narrow angle of view and the shallow DOF). Sure, you can zoom out with your feet, as they say, but then you're going to have audience member heads in the way. Some of my other most treasured shots have been with the very expensive 14-24mm where at its widest, I could get the whole of a 5 piece band pretty easily and can produce some interesting effects if I'm up close and don't correct the distortion in post.
Since you already have the 50mm and can get up quite close, I would either look for a fast 24mm or 28mm prime used, or if you can stretch a bit and can get a good deal, get the 28mm f/1.8g new. Or perhaps even better, get the Tokina 11-16mm (which will give you results on your D7000 similar to the results I get from the 14-24mm on my D600). The 11-16mm is a fantastic, sharp DX lens which I used to own and sold when I traded up to the full frame D600. Even though I don't really need this lens anymore, it was so great that I still regret selling it. It's an f/2.8 and so not as fast as many primes, but it has a fantastic angle of view for getting the whole stage, or audience members up front dancing, or if you can shoot from the side, you can get the band or DJ and people dancing in the audience all in one shot. If the lighting is as poor in the clubs you'll be shooting at, you'll have to do some serious adjustments in Lightroom afterwards, but you'll get some amazing shots with this lens. I bought mine (with caps, but no box or receipt) in excellent condition for $500. I later sold it for exactly the same price and that's within your budget.
I have the Nikon 16mm fisheye as well and so can answer your question about that -- it's not nearly as useful and adaptable as the Tokina 11-16mm. Don't misunderstand, with practice and after getting to know its limitations, you can get some really cool shots from it, but it has a very limited range of uses. Friends warned me of this when I bought it and they were right -- I don't use it nearly as much as I thought I would. And if you're hoping to use some of your shots for print or online live music reviews, you'll find that it really doesn't produce that many suitable images.
Hope this (really long) answer helps somewhat!! Have fun!! Shooting live gigs is an amazing experience!!