I run the upper Potomac in the Washington County area, so this might not really apply, but I have a 1648 mod. v with a 1965 Johnson 5 h.p. I know the 5 h.p. seems small but it works REALLY well in the river up this way, which is pretty skinny. I can run full throttle through the shallowist parts and just let the motor bounce off the rocks. It's pretty darn slow going through parts like that, but unless you have a jet, your not getting through those parts with a higher h.p. prop. I want to get something bigger, but can't justify it when I can get 3 guys and a boat full of gear to and from my spots without issues. Now I only go 2-3 miles max from my put in. If it were any farther than that a bigger motor would definitely be worth it. Alot of guys up this way run jets, and they work really well for this part of the Potomac.
I like the 1648 size, but something bigger would certainly be nice. I would think for you a 16' would work well in the smaller areas, but still be big/stable enough to hunt the bigger waters. If you run any skinny and rocky water, I'd stay away from the mud motor. Mud motor + rocks do not work out to well, IMO.