A Di unit will not replace a standard 2d unit. The Di is nice to see if that blob in 2d is fish or brush. The arcs in the standard 2d is what the pros look at and is more accurate.
I would save up and get a HB Si unit with the biggest screen you can afford/fit.
I have an HB 898 si and like it. My friend has a Lowarance HDS 5 and I dont care for it. The 2d in the HB and Lowrance are both good. Si in the HB is much better than the Lowrance IME. Di is better in the Lowrance, but as I said, I dont use that screen much.
If you store any unit on the boat and under a cover they can fog up from the humidity.
....Bill
...Bill