advices for portrait shooting with SD15
I've used the 17-50 F2.8 OS, and it is quite nice. For portrait work you might want to stick to the 50mm end of the zoom range. The OS works pretty well on that lens, but a tripod would eliminate uncertainty.
If the 70mm is sharp, it would be even better. Definitely recommend a tripod for any lens that does not have OS.
I've had good results at high ISO in low light with my SD15 using the 85mm F1.4 handheld, so luck plays a big part in it.
Take quite a few more shots than you think you'll need, then sift through them later. What usually happens is that the best pose is blurry. Goes with the territory, I guess.
Good luck, and above all have some fun.