Cheapest long/fast options I can think of, depending on what kinds of concerts and sports and hence what sorts of distance/lighting. If f2.8 is fast enough and/or 85 to 100mm or so is long enough, you've got some pretty decent options available.
180/2.8 AF-N ED version. Goes for £200-300 for E+ used. One of the best lenses Nikon ever made too, in terms of sheer image quality, but maybe quite not fast enough for your needs? (I'm told the D50 is pretty decent at high ISO though.) I think it's at least as good as the 70-200 VR in terms of sheer image quality, although obviously it can't compete with it for convenience except in terms of size and weight. It's so light you can pack it just in case.
85/1.8 AF. Goes for about £200 E+. Fast, maybe not long enough, but nobody ever complained this one wasn't very sharp. The 85/1.4 is a bit better shot wide open, but is also about x3 the price and it's certainly not x3 better.
80-200/2.8 AF ED (push-pull version) very decent zoom for about £200-300 quid in E+ condition. Reputed to suck in dust due to push-pull design, but the newer AF-S versions that don't suck dust, are x2 the price or more.
105/2 DC. Awesome professional portrait lens. Would work very well as a concert and sports lens assuming that 105 was enough range and it'd potentially give your stuff a unique 'look' that'd make it stand out from the crowd. Since the new 105/2.8 VR micro came out, I've been seeing a few of these going used for under £500 in E+ or better condition.
