It's abit of a minefield - I have a requirement for more blue ones now and this week one of the ' 31111' variety went for over £100 on eBay.
the thing to watch for is not only the chassis, but the screw columns in the body become brittle and break off when the screws are loosened or titened.
You would have thought a bog standard BR blue 31 would be in production for ever, but ewhen was the last version actually produced ? 2008 ish ?
From memory we have 31270 ( original, flawed ), 31174 ( revised body with strap, some iffy chassis), 31111, 31165 ,31268, the last three being fine