I'll let the others elaborate on most of your questions, but here's my on what I've read :

Humminbird has the best SI, but their DI is an interpretation of the SI image.
Lowrance has the best DI, as they have a dedicated crystal strictly for DI imaging.

Most all the upper grade units will do SI/DI/2d.

Personally speaking ... I have a Lowrance Elite 5DSI mounted on TM, and I don't miss the 2d (the E5DSI is down scan only). My unit has been discontinued by Lowrance, and 2d added ... now called the Elite 5HDI.

According to a lot of members here, "Brian@BBGmarine" can give you the best deal on whichever unit you decide on ... and can even suggest the best unit for whatever purpose you intend to use it for, or where you mount it.

... cp