Shun Cheung , Aug 05, 2005; 12:18 p.m.
In you case (FM2 body), just make sure that you don't accidentally get a G (or G-equivalent) lens that has no aperture ring. Anything that is more than 3 or so years old should be safe in that sense.
Otherwise, could you tell us about your budget and what your photo subjects are? Unfortunately, most of us are more familiar with used prices in the US rather than in the UK, but I am sure some can help you.