I know everyone loves the nifty fifty, but the 35mm f/1.8 is a much better and far more useable lens for crop sensor cameras, but it is around £150-190, the 28mm is even better, alot sharper than both and some of the nicest colours I have seen including L lenses, but it is £250-300. Although this is the lens I use 95% of the time so worth every penny imho.
They will all deal with low light suituations without the need for a flash, although I find the 50mm too tight for using indoors really. But good for taking pics of faces only.
If it were me I would see what price Kerso could do the 40D kit for with the 28mm or 35mm prime.
Or just use the kit lens for a few weeks and then decide what area you need to improve on, without knowing what sort of photos you want to take it is hard to know what to buy. No point buying a big zoom if you get into street photograhy and people, and no point buying a really fast prime is you want to take pictures of birds etc.
I would get the kit and take it from there.