The helix is about the cheapest si unit you can get right now. Not sure of the reliability of it due to how new it is. If all you care about is DI and SI but not screen size, that is probably the way to go. If you have a bit more money maybe a used 899 for something with a little larger screen. Im not exactly sure but I think about the cheapest you can get with a new Lowrance with SI is around 1400. I bought an elite 7 hdi with di due to being able to use insight genesis to make my own maps. It all depends on how much money you are willing to part with and what you want to do.
Good luck.
Whatever you do read some owner reviews on whatever models you have in mind.