Jon, there is a shortage of little outboards at the moment because its the height of the season, so if you could wait until Oct or Nov then you might find a bargin. Just been looking for the same thing and ended up buying a new one from Fairweather Marine, but E Bay and Boats and Outboards would be a good start point.
http://www.fairweathermarine.co.uk/
As for the boat, well the small outboards are about 12 kgs so shouldn't worry about the transom if you have the room. You could use a block of teak or bracket to raise the aux up slightly.
Pete