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If possible, plug it in and get it playing. Use your own cartridge and records so you know what it should sound like (no warbles or warps unless in original track) and check the arm tracks properly throughout the possible range (one of mine can float a bit with 7s but is fine with 12s). Play with it a bit too if you're going to be mixing/scratching/etc - they're pretty robust but a bent tone arm isn't always obvious until you've seen how it tracks under manipulation.
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It's very likely fine. They are pretty bombproof. There's a difference between a working deck and a properly set up one but most problems can be solved for reasonable money. If you're the kind of person who needs things to be 'perfect' then the new SL GR is £1100 so that's a consideration if budget allows. Otherwise it's a lot of quality for little money in record deck terms.
Varies quite a bit based on condition. £300-£400 seems to buy a reasonable looking one but you have to bear in mind they are almost all in need of some tlc. Worst thing that could happen is some twat has played with the arm bearings, otherwise it's main bearing oil, resetting pitch sliders, new led light bulbs, cartridge connector screw tightened, cosmetic damage, missing lids etc. Luckily most of the parts are easily available these days.
Personally I have the Mk3D with the pitch reset button, it just works better for me but some people miss the centre detent on the pitch slider.