Assuming there's not a strong head current/wind, a 30lbd thrust should be plenty. I use a 30lbs Minn Kota C30 on my kayak off of La Jolla with no problem. My top speed is 3 MPH with a current, and 4.2 MPH without a current/wind. Note, a higher thrust motor will not nessacerily get you higher speeds unless the motor is undersized for the boat size. For your boat, anything over 40lb thrust is overkill and will NOT result in a higher top end speed.
The challenge is finding the right battery for your application (range/distance and battery weight limit). I average around 5-6 miles on a 40 AH SLA battery. You will actually get quite a bit more range if you run at slower speeds. I recently switched to a 40AH LiFePo battery (lithium) and will never go back to a SLA. They are so much more light, smaller, and actually go about 20% farther than an equivalent sized SLA, but they are EXPENSIVE. If you do use it in saltwater, don't bother paying for the more expensive saltwater version. Just make sure you wash it down thouroly after each use and it will last forever.