I put an 18lb watersnake on my 3m kayak and it works a treat. Youtube search potatoes GDITB and there is a video on it.
spec wise, i have a 38ah battery which is good for a 4 hour session of regular but not solid use. Going off the gps on the sounder, it goes 5knots +- wind and tide. I initially had it mounted off the side, but this was inefficient and uncomfortable to use. I had to use the rudder to counter the off centered push of the motor.
After a couple of trips i decided to make an aluminium bracket for the motor to fit in place of the rudder so i can stear with the foot pedals. I pulled the switches off the motor and used extra wire to put the switches in a small box next to my seat. I have the battery in a bracket which has survived one capsize.
for a larger kayak i would definitely go a higher thrust engine. Also check and make sure to use appropriate gauge wire for the current the motors draw, use water proof connectors, and a fuse. Ive used mine over 50 times and find it quite low maintenance.