There won't be many used bikes with the power you need. Even if the motor's OK, the battery will probably be worn, which reduces the power. £800 gets you a new Woosh Big Bear, which is probably the only suitable new bike. If you're really strapped for cash, you can find a Powabyke Euro for £100 upwards with knackered batteries. A set of decent batteries is about £120. They're heavy but have high torque.
All Ebikes will help you more than a non-electric bike, but don't expect an average 250w motor to drag you up steep hills.