I think your questions need more defining - such as - what is "reasonable speed"? Also, do you need this propulsion in calm water or rough water or currents/tide?
There is a fantastic thread on the comparison of the options for propulsion for a TI -
https://www.hobie.com/forums/viewtopic. ... 64#p334642 Short story, in his testing a trolling motor was so worthless he didn't publish the results and he makes the case for why a a 3hp equivalent electric outboard is by far the ideal propulsion solution. This was helpful for me to choose the Torqueedo Travel Ultralight, but there are other options and he reviews them extensively.
To get a realistic goal, I would recommend calculating worst case scenario - what is the worst case max distance you may need to cover? And, what is the worst case scenario for the amount of time you would have to cover that distance? Finally, what is the worst case weather (wind/current) that you may need this propulsion to cover?
So if you are ok with short distances, and relatively slow moving through calm water (ie, ~3mph), I think I recall that a 55-85lb thrust trolling motor is minimum, but I cannot recall where I read that.
But, for me a trolling motor was useless - if the conditions are ideal then I will sail. If the conditions are a bit less than ideal then I can pedal and/or combine with sail. But the conditions are such that I really need to motor, then I need a "real motor" - I decided to go with a Torqueedo Travel Ultralight (3hp equivalent). This can push the boat to 6+ knots if needed, which is enough to "get home" against wind, current, etc for the distances I need.
hth,
Brad