Assuming the bike is being sold legitimately I reckon it's hard to judge a fair price.
A new battery and lock would be £700, but the bigger problem is there's no way to test the motor.
You could put a borrowed battery on the bike, but as soon as the battery clicks home there's no way of removing it without the key.
It might be possible to blank the locking mechanism and still wedge the battery in place sufficiently firmly to make electrical connection.
Buying the bike without firing up the motor puts you at risk of a total recommissioning bill of about £1,500.
With that in mind, paying anything more than £1,500 for it doesn't make much financial sense.