Ill second what Ander had to say although I don't know about them new fancy 360 and panoptix thingys. I started with a little elite 4 on my kayak that had DI. Paired with the sonar, it was great. I picked up a power boat a few months ago with a hds5 gen 1 up front so I stuck with lowrance and added a hds7 gen1 on the console, lss1 and linked them. Its 6 year old technology at this point, but from gen1-3 and lss1-2 they have not changed much. Being able to scan an entire cove in one pass rather than zig zagging all over it is huge.
That being said, it isn't like it gives you a VR headset and lets you virtually walk around the lake floor looking for fish (that's probably be here within 5 years though). Being able to (1)configure everything so it works properly to get good images (2)take what you see on the screen and visualize and place where things are in the water takes the same type of skill as with your old sonar. So, if you don't really do much of that now and just use it for general depth/temp, you probably wont do that with side scan either.
As far as the actual units, I am the type of person that prefers top of the line used things for the same price as new low end things. I can talk lowrance on the HDS units and the elite, but don't know much about hummingbird or garmin. The boat I picked up had a gen1 and some basic wiring, so adding hds7 and being able to link them was my choice. Keep in mind those damn proprietary cables are expensive. $30 for a power cord, $70 for two for lowrance Ethernet cables, $70 for ram mount in addition to the bare unit cost. If this guy pulls all the wires, this would make a good package -
Gen 1 package, especially with the puck and nmea wiring. Dual units are nice, but linking them is a must for me. They can share map cards, waypoints, transducers etc. I don't believe you can link the lower end units.
Ill also second installing it yourself especially if you are straight up replacing an existing that was done right the first time. Like MattATX said, just attach you new cables to the old ones when you are pulling them out. Running new wires is typically the difficult part especially on an older boat.