FWIW, I had decided ages ago that I "interacted" best with Lowrance kit having blagged a play on various friend's boats. Like all electrionic gubbinry, my fave will be the next bod's worst nightmare....
Having decided Lowrnace, I basically sat & watched ebay / Ribnet bits'n pieces until what I was looking for showed up "preloved" & within budget. What I ended up with wasn't my "Money no object" ideal (I wanted the version with the built in GPS antenna, to keep the clutter off the A- frame) but as I now have an unplanned NMEA cable runing to the back of the boat & a plotter that has "engine data" capability (Stuff that wasn't on my original wants list) I am now looking at installing cooling pressure & cyl head temp senders which will plug into the cable I have already laid nicely, and give me a degree of engine monitoring that wasnlt even an option when Merc built my engine!
Thing with plotters is that a new map set is going to add a good couple of hundred quid to the price, regardless of the machine you bolt to your console, which is where a used machine may work in your favour and come with the charts.
As already mentioned, sounders may or may not be a simple "plug in", although a newer NMEA2K plotter should take a depth input from anyone's NMEA2K sounder module. When I had narrowed down my options, one was the "plotter only" version of what I ended up with, the theory being I spend about the cost of a fishfinder (which I have no console space for anyway) on a 2K sounder.
FWIW, one of my deciders was I prefer the look of the Navonics charts, so a Navonics compatible machine was a must. - If that sort of thing doesn't bother you, the field is fairly open. I personally would aim for something with NMEA2k capabilities, which will if nothing else help to "future proof" it a bit.
Back to the ribnet massve for stories of durability!