For 95% of what I do, the "fishfinding" is more a navigational process than an active "finding".
the two major locations here are wrecks and grounds and techniques are drifting or anchoring.
coming to your question...

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the two major talents of the fishfinding are accurate plotting with pre-loaded maps and ground imaging.
nowadays technology develops all the time and I believe the GPS plotters with 3D maps are nowadays affordable..I would love to have one at least for ground fishing and uptiding.
the sonar function is usually quite a primitive device and largely only used to know that we actually are on a wreck. If you have enough marks on your plotter you can do wreck fishing without a sounder imo.
other underrated value of those devices is measuring of sea temperatures, tracking, voltmeter, VHF/GPS link, split screening, zooming
after all this, the only advice I can give is: buy as good as you can or want to afford, but the plotter is the really important thing
dont buy old and outdated stuff
also check connectivity (PC, VHF, other devices, multiscreen, Iphone/Pad) and touchscreen/controls