In reply to MikeR:
I can't help too much with the lenses for a crop frame (and you seem to have got good advice here already) but if you've not already have a look at:
http://www.ukclimbing.com/articles/page.php?id=6741
There really is very little little need to stack photos unless you're doing deep space photography (in conjunction with a sky tracker) or unless you're planning on creating star trails. The simplest and best way to deal with noise is to remove it in Camera RAW (PS or LR) using the luminance noise slider (zoom in to at least 100%), 50% normally works well without compromising sharpness too much.
Feel free to experiment with 'long exposure noise reduction' (the dark frames you were talking about) but in my experience it's a mixed bag, even with an expensive camera.
Have fun!