The 5" Humminbird Helix is hard to beat for the money.
Everybody preaches bigger size but that not required to get excellent basic structure and coordinate location.
The good news is you can pick up a used,
like new unit for $150-$250 on here or similar places.
So you can be in business for half or less what even the cheapest 7-9" units run and see how you like it.
Then you can see how you like the unit and if you decide later you want bigger or better you've got an excellent 2nd unit to accompany it.I