I use a 45lb thrust Motorguide 'leccy motor, and its excellent for trolling - but I've only used it on glass and wooden boats on freshwater - not on my Rib.
Have you ever tried to paddle your Rib over any sort of distance? I have run outa juice about 1000yards offshore on Loch Lomond, and paddling to shore (two of us) against a cross-wind took forever. A Rib catches any sort of breeze, and if that breeze aint going in your direction then I think you may be asking an awful lot of an electric motor to get you home (eventually).
Just my experience, and in my opinion your money would be better spent on a decent small 2-stroke. It would still be suitable for trolling, but would provide a coupla more horses for getting you home in emergencies. The bonus would be that you could use same fuel, and you aint gonna run your batteries flat.
Hope this helps...
"Remember the Plug!"