There was someone on here the other day that got caught out on a new (second hand) car that was involved in a crash, he was left £1k out of pocket. So it can be advisable on any car purchase.
Check the value on http://www.webuyanycar.com/ They will give you a low price that should be clsoe to what the insurrers will value it at.
Its all depends on your attitude to risk, risk adverse then go for it. If you are happy to take the risk of having a crash and losing out if its a write-off then dont go for it.
And shop around you might be able to get gap insruance for cheaper elsewhere.