what to look for in new tank?

I hope the crosspost is ok; it seems more efficient than posting an
identical article twice.

Tomorrow I'm going to look at a 48 * 18 * 18 tank - it's very cheap
(£20, which is around $36, and a fair bit less than half price in the
shops). I think it's new - the guy selling it is a _serious_ fish
hobbyist, makes his own tanks and has taken voluntary redundancy to
start his own aquatics business - he says he's selling to clear out
space in his unit (and has lots of other "bits", too...) He seems like a
very genuine, friendly kind of bloke - he says he can give me a lift
back with the tank in his van, which is astonishingly generous (he must
_really_ need that space...).
What I need to know is, as he isn't an established business and as it's
the kind of rock-bottom price that probably comes with very few
guarantees, how can I spot a duff tank? I know too make sure all the
corners are flat to whatever surface they stand on, but would any faults
in the sealant be obvious? Should I look for anything else? - once I get
it home and fill it up with water it could well be too late. The kitchen
ceiling will not withstand 67 gallons of water coming through it!

Any suggestions will be gratefully received!

many thanks,
--
sophie