Hi hamsty!
As already said, there will be no difference in quality assuming that the EOS 300 is working OK. The more expensive film SLRs like the 50e had additional features, and more metal and less plastic in the body, but the lens and the film are the critical components.
If you do buy something like the 17-40L (recommended!) you'll also be able to use it on any Canon digital SLR if you make that move in the future, although the field of view will be different by a factor of 1.6 on all but the 5D and the 1D series.