I would always endorse the idea of having at least one fast prime like that...if you can find one that gets you in the door for under $100 or so, that would be exactly what I'd recommend to start with. If you find you use it a lot, and really come to love low light shooting with it, but find it a bit too long, you can always look to invest in a good 28mm, 30mm, or 35mm version down the road. But a cheap 50mm is always a great place to start. I myself got a Minolta 50mm F1.7 as my first low light lens, picked up used for $60. I'd suggest the same - you might even find that lens used for even less...check out KEH.com to see if they've got any.