I'd recommend the Tamron Adaptall SP 300mm f2.8 LD IF (60B model). Stick a Tamron Adaptall SP 2x BBAR teleconverter on it and you get a 600mm f5.6 with virtually no noticable loss of sharpness and very well controlled CA.
I picked up mine, used but with mint optics from Jacobs in London for about £300, (reduced from £650!!!). Not sure what they go for on ebay but if your lucky you might get one cheaper.
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