If I read this correctly, you are seeking info on GPS/Fish Finders????
I highly recommend spending the $$ on a GPS. Each time I go BTB I find myself trying to figure out at what point I launched at the beach. The blue 2-story house doesn't look quite the same 1 mile out as it did when launching:) I have also found out there are several blue houses along the beach
You don't need to spend lots of $$ on a GPS. All you really need is the ability to follow your "tracks" back to the launch. Of couse, waterproof is of utmost importance. No need for a compass... every gps I have ever seen has one. The one I got is a small handheld (Garmin Dakota). Very satisfied so far. PLUS.... If your secured (?) battery doesn't knock a hole in your kayak, a deck mounted FF will probably be torn off when your kayak is tumbling upside down through the surf
Fishfinders are over-rated in my mind. I only use mine to see the bottom contour and the depth. You might be able to see some schools of fish in water over 20 ft deep. The sonar cone really doesn't start showing fish until you get to that depth. When BTB fishing kicks into high gear, your not going to be looking for schools of fish on your FF. Your going to be holding your head high looking for busting pogies schools

That's where the big fish are hanging out
Also, try to figure out how your going to store you gear when going through the surf. I don't care how many times you have practiced. The surf is going to kick your butt sooner or later. Having your rods broken, reels full of sand, sunglasses/etc gone can get very expensive.
Can you store your rods/reels inside your kayak?
Do you have leashes on items your not able to store?
Take only the bare minimum of equipment!
Good Luck!