Whilst being a cheap lens, the 50mm f/1.8, when stopped down to f/2.8 or f/4 will give very acceptable results.
A lot of my early imaging was done with the sturdier 50mm f/1.4 stopped down. Both will exhibit horrible coma and spherical aberrations out at the edges, but, you can always crop that out.
If/when you can afford it, go the L-series route. The 200mm f/2.8L II USM would be your best bet. Although, for the price you'd be able to pick yourself up an ED80 and an ST80 for autoguiding.
Regards,
Humayun