I have a 12.8ft kayak, looking to outfit it with a trolling motor. What size transom would be best? 30#, 45, etc? It weighs 100lbs dry and with gear and me it's about 350lbs.

I have the same kayak.  I have a 30 pound minn kota endura,that I strap on it occasioally. It will scoot along pretty fast with that motor. If I am using the Kayak it is usually without the motor.

You have to be careful with a motor on a kayak. sharp turns at high speed, can tip you over.

Thanks for the response,  I will be using it 99% of the time on the lake in cove, so hopefully closer to the bank. I appreciate the info.

I have a 12T, also with a 30lb trolling motor. I could be wrong, but from what I understand, a 30lb, 45lb, 75lb, etc, will all get you about the same top speed. The higher number just gets you up to that top speed faster. Best decision I made with mine was mounting it on the stern and running cables up to the foot pegs for hands-free steering, and wiring a control box up by my seat to control the speed.