You might just get a 100-400 used for that price - which of course has enormous flexibility. If quality is most important, then the 70-200 f4 is extremely good - the new one with IS
would be first choice, but I'm not sure how easy that will be to find used, The old non-IS
version is also very good, and used should be relatively inexpensive (easily within your budget).
If the flexibility of a zoom is not important, then the 300 f4 is excellent value for money - it aslo works well with a 1.4 convertor.
Colin K. Work, Pixstel