Essentials - a good rear 360 degree safety light and a comfortable PFD.
The rest is my personal opinion - I would not go crazy with interior lights. More lights near you, more bugs near you. Wearing a typical quality led head lamp for use only when needed seems to work fine for me. External lights for safety or shoreline viewing at night. Go simple here as well. I had some 40" 60 led strips in Blue color on my original PA14 and it lit up the water and shoreline awesome however if anybody fished near me they were blinded. You can pick if that is a problem or not.
To view shoreline now I am going to use some small white LED strips or small floodlight on a RAM mount off the front handle. I typically like to pitch toward the bow so I feel some spread lighting up there will do the trick. Plus any bugs attracted will be up at the bow and not on me.Batteries need to be sized by your current draw and time on the water. Figure out entire load / draw and multiply by the hours of your trip. Say your finder draws 1 amp and your leds draw .5 amps x the 6 hours you want to fish 1.5 x 6 = 9 amps. Plus you never want to drain a regular AGM or sealed lead battery all the way. So I would shoot for a 12-15ah battery to be safe.
Adding a TM to the mix means a way bigger battery depending on the TM you get and how fast and long you run it. If you really want to run a TM all day, then you pretty much will need a full-size marine type battery.
Fishfinders - get the biggest darn screen you can afford. Especially if you want to view side scan or double/triple panels at once. Side scan on a 5 to 7" screen sucks. I mainly use regular 2d sonar and GPS / mapping. But I am older and so are my eyes so I want these things as easy to see as possible. I miss fish on the side scan on my HDS7 all the time. I see it later looking at Sonar logs on my PC. Hard to see those little white marks out in the sun on a small screen. Plus you have to be moving consistently and fairly level(no rocking) to get the best picture for Side Imaging. Not a simple feat in a yak on a busy lake. Therefore I rarely use all the goodies I paid for.
Side scan and some downscan transducers also bring the challenge on where to mount the darn ducer in the water with clear line of sight(for sidescan). There is no easy fix for this. Just pick your poison with either side mounting via some arm, or rear/ front mounting off the handles. You have to deploy it, retrieve it when beaching or exiting, deal with drag or weeds getting caught, blah blah blah.. Stick with what fits in the lowrance ready and you will have less hassle in the long run. If you waste 10 mins getting ready to launch and 10 mins getting out, you have lost 20 mins of fishing time every darn trip. Keep it simple and effective and enjoy yourself.