Complex sentence: As it was not expected to encounter fighter opposition, high performance or manouverability was not considered important, but long range and heavy armament were, with the provision of a navigator/wireless operator considered essential for the long, over-ocean flights would be required.
Split into simpler sentences: As it was not expected to encounter fighter opposition, high performance or manouverability was not considered important but long range and heavy armament were. The provision of a navigator/wireless operator considered essential for the long, over-ocean flights would be required.