If you only need it for days with little to no wind, and very little or no current, then I'd say a 2hp HONDA will do. They make it in a longshaft and it has a cetriugal clutch (no lever, when you advance the throttle it auto-shifts) and at under 30# it is a lightweight. However, if it were me, I'd tend more towards a Mercury 3.5 hp or even a Merc 4hp both (as far as I know?) are available with a longshaft and I think the 4 will have ,at least as an option, a remote fuel tank for greater range. Nissan, Suzuki and Yamaha all make basically the same outboards as Merc in these sizes. Honda also has a 5hp. All of these are 4-stroke outboards. 3.5 to 4hp seems to be what a lot of Mariners have, mant have a 6hp and some as big as 9.9, I'd think anything over 5-6 would be too much power to really use. Under 3 hp might be too little depending on wind/current.
Unfortunately, the Mariner 2+2 doesn't have a good, ventilated place to stow a fuel tank other than the cockpit footwell, but at least you don't need a bracket for the outboard, it can be clamped right onto the transom next to the rudder.
Another option is a good electric outboard, about 20-30# thrust or more. It might not handle a lot of wind and/or current but it calm conditions they work well!
I have a 4hp Johnson (2-stroke) on my DS II and feel that it gives me what I need with a definite "reserve".