I’d look for two used combo units, but would buy new networking cables and the 5-port ethernet switch unless someone throws them in for free with the units. (Be careful not to get an “InterLink” as that’s older tech.) For me, saving $15 on a $50 cable isn’t enough to risk used connections that are kinked, etc.
If you do some searching you could definitely get an 1199 SI and an 1159 sonar plus the 5-port ethernet switch and all the cables you need for under $2000. These aren’t supported with updates anymore (after Dec 2016), but it’s sound so you won’t need any more updates.
The slightly newer used Helix (generation 1) units run on the same network parts, you’ll just have to settle for slightly smaller screens for the same money. Probably a 10 and a 7, or two 9s. Smaller screens for same price, but brighter and include the option to use AutoChart.