I've been down this road. Whether you buy a trolling motor kit (Bass Yak, Island Hopper, Kevin Dismuke, etc.) or build your own the cost is about the same (assuming you're talking about a rear mounted TM). The big cost is the battery. You can get a Walmart battery that weighs 65 lbs and lug that around if you want but consider what weight does to a kayak. Even adding 20 lbs of extra weight makes a huge difference in the kayaks efficiency. Adding a 65 lb battery plus 20 lbs of trolling motor & mount could equal an overloaded kayak. You can go lithium but 100 ah lithium batteries start at about $700 plus the charger. By this point it's about the same cost and a lot less headache to get a Bixpy. Lightweight with equal or better performance than what you will get from a standard trolling motor.
Not trying to talk you out it just letting you know about the things I didn't think about and realize when I did my TM project.
Thanks. I appreciate the insight. Were you ultimately happy you added a TM?
It wasn't worth it for me. Carrying a heavy battery back and forth from the truck to the launch got old quick since the battery was too heavy to put in my kayak with the cheap cart I had. Also the kayak I had didn't have the weight capacity to hold the extra weight and the rear tank well would have about 2" of water in it the entire time.
I ended up getting pedal drive kayaks and love them. I enjoy the excersice. I have issues with an elbow and lower back issues so I can't enjoy paddling like I want to which is why I went the TM route to start with. If I ever went to an electric motor again it would be a Bixpy without question unless lithium batteries drop in price by about 70% lol.