Any of those units are good, except for the limitations of the Lowrance DSI unit (down scan only). While it does produce good images once you learn how to use it, it is worthless as a fishing tool. It is a good companion unit for a regular sonar unit to get you into the imaging technology and interpret what 2D sonar is indicating.
You can get those units at a lower price if you shop on-line and not a a large general sporting goods store like BPS, Cabelas, etc. Just do a web search for each one.
Turning the trolling motor with the down imaging technology doesn't distort the recorded images like with the side imaging technology. The deal with imaging technology is movement has to occur for the unit to "draw" the image and the shape of the sonar pulse is very narrow front to back, but wide to the sides. That makes tracking vertical presentations just about impossible. You need a regular sonar cone pulse for that and a dual frequency unit with a narrow and wide cone is the best.
I use Side Imaging, Down Imaging and 2D sonar at the same time and the dowm imaging is the least useful for fishing.
If depth is all you want, then any of those units or even a lesser unit will give you a digital depth reading with some sort of recorded image.
From your list, the 570 DI is the top choice.
The Hummingbird 768 combo is a discontinued unit that retailed for $500 when introduced. If GPS is a wanted feature, then that unit at that price is great. That is why is has a sale price at BPS.