I just recently bought a 27 inch 2560x1440 iMac so I can do OSX development as well. Surprisingly really cheap. It was $1999 which was less then I was expecting, especially since I can use it as a 2560x1440 display on my PC =P. But for your actual question I would really just buy a new one from apple, it is a little bit more but it will be better in the long run.
I got my macbook from CEX for £500 a couple of years ago, it had a dead battery hence the cheap price but you might be able to find some other good deals.
What about a Mac Mini? They are pretty capable little boxes. The base model is only $599. Buy some memory for it and you should have a decent platform.
Have a look on Gumtree, maybe someone local or willing to include postage will have one about for a decent price.![]()
you may also find these links of interest:
http://forum.overclo...le-macbook-pro/
http://forum.overclo...macbook-pro-15/
http://forum.overclo...-32ghz-core-i3/
I have a Imac g5 17" for sale but as i don't have the box for it, posting would an issue