My suggestion would be a 5 inch or 6 inch f5 newtonian. This would keep you within both your price constraints and the weight limits of the HEQ5. These are two options, both with 2 inch focusers which would be mandatory for imaging:
5 inch skywatcher newt:
http://myastroshop.com.au/products/d...sp?id=MAS-069A
6 inch skywatcher newt:
http://myastroshop.com.au/products/d...p?id=MAS-069A2
I have previously used a 5inch newt for DSLR imaging on a HEQ5, with very good results - see astrobin link
here