We have a Tohatsu 6hp single cylinder 4 stroke long shaft, weight 27 kg which is quite manageable single handed although more of a struggle in a seaway. We sail and motor a lot in company with another TS240. He has a 9hp 2cyl 2 stroke Merc which is much heavier. He uses about 4 times as much fuel as we do so has all the extra weight of fuel as well as the heavier engine.
The engine will just about manage 6 knots. The disadvantage is that a single cylinder engine produces a lot of vibration which is quite wearing on a long windless trip. We motored back from Cherbourg last year.
We chose long shaft to ensure the prop never comes out of the water even in quite extreme conditions.
We have also experimented recently with using the tender outboard, a 2.3hp Honda 4 stroke, weight 13 kg and can be lifted in and out with one hand. This will achieve over 4 knots in ideal circumstances but is soon knocked back by a head wind.