A common inexpensive choice for beginners is the 1100D, they work well for astro and there's plenty of useful software.
The 1100D has higher ISO than the 1000D or 450D, it's no longer made but still available as clearance stock from some major outlets
Example...
http://www.harveynorman.com.au/canon...nly-black.html
Or have a hunt around and grab a second hand outfit.
Good luck.