For planets and Moon, consider web cam.. see here:
http://www.qcuiag.org.uk/
For other, deeper things, second-hand DSLR is the option.... old Canon or Nikon could fit into your budget.
However, most dobs are not good for longer exposures (primarily due to lack of field rotator)...
Also you have to see if you will be able to reach focus with your Newt.
If you are handy with tools (and heart
), this is not a problem. But if you are not... it may be quite expensive exercise.