As far as im concerned, at least for how ( i like to fish ), i could do without a fish finder for Crappie fishing.
I do have one, its a Humminbird Helix 5 model, which i simply use for depth
It also has the down imaging feature which i never use.
If i were to buy one again id buy the one having side imaging, simply for finding beds.
Most of the time im not paying attention to the fish finder at all.
Mind you, i said the way i like to fish, which is using a long rod with 5 or 6 feet of line hanging from the tip.
I mostly use either a 1/16 or a 1/24 oz jig with a bobby garland baby shad grub, or something similar.
Mostly i have a small split shot pinched on the line about 6 to 8 inches above the jig.
I go very slow along the weed edges, almost no speed at all.
Also dip the jig into holes in the weeds, and just wait a few minits before moving on.
With the shallow lakes i fish they are in the weeds all year long.

When i first started i hired a guide to go with me in my boat.
He brought with him 2 telescopic 10’ BnM poles, and about 6 of the cheap Bass Pro shop jigs that cost about 3 bucks for 10 of them.
We caught 17 keeper Crappies, and he probably caught 12 of them.
He took me to a place along the St. Johns River near Melbourne, and we fished the way i just described.
Two days later i went back alone to the same place, and fished the same way, and i caught 14.
But this time i was in front of the boat, not him.

I still watch quite a few You tube videos.
But i dont watch the ones where the fisherman is out on a lake staring at a fish finder screen.
No doubt they are catching fish, but that type fishing isnt for me.
I also dont watch trolling videos.
I have tried trolling, both out the back of the boat, and in front of the boat, (spider rigging).
And that can be successful as well, provided you like fishing that way.
I will at times troll a couple rods out back when im moving from one spot to another.
And i often have another rod in a holder on the opposite side of the boat while im moving along the weed beds.
Not trolling with it, just a few feet below the surface.
I rarely use minnows, mainly because i have to go out of my way to get them.
So thats how i do it as for Crappie fishing.
And if it works for me it could work for anybody.
If i werent so far away, you would be welcome to go along with me a few times.
But try it the way i described doing it, and see how that works for you.
Theres a guy on youtube by name of ( Brent Allen ) he fishes the same way here in Florida, and he does very well.
Check out his videos.