Originally posted by mike star The standard 18 - 125mm lens that came with the camera has been OK for general outdoor photography but as soon as I have atempted to take indoor shots from a distance like at my daughter's basketball or school functions I get really poor results (hand held) as the lens just doesn't seem to work well at all in low light unless I could set up and use a tripod.
So I assume you mean, you couldn't get a fast enough shutter speed to eliminate blur. As another poster mentioned, that's because it doesn't have a very large maximum aperture. Although you should at least be sure to turn the ISO up as high as you can stand, and make sure you are using the largest aperture the lens does support (f/5.6 when zoomed all the way to the telephoto end). Consider underexposing the shot if necessary to get even a faster shutter speed, and brightening the picture in post processing.
I doubt either of the older lenses you have would be any better here. A lens with large maximum aperture (smaller f-number, like f/4 or f/2.8) is what you need. If you can handle manual focus and don't need the lens to zoom, you can get used 100mm, 135mm, or 200mm in that aperture range lenses for very little (under $100 each). But it you want autofocus and zoom capability, then it will cost a lot more to get a lens that is "fast" enough.
Quote: Would an 18 - 55mm for example be a good starting point?
No. Not only is it no "faster", but it only goes to 55mm, and I'm assuming you are using the 18-125 more toward the 125mm than the 18mm end.
Quote: I have just found a very good condition used Sigma 18 - 50mm f/ 3.5 - 5.6 DC lens for sale for under $200 AUD here in Sydney. Would this be a useful lens for me to grab now?
No. this lens is pretty universally considered to be far worse than the Pentax 18-55, which costs no more. But really, neither of these would give you any advantage over what you've got. If you need faster shutter speeds, you'll need a larger maximum aperture at the focal lengths you actually use at the games, and I'm guessing that's not even in the 18-55 range.