I have two early model 898's. I'm not sure what you mean about not being usable under 2 mph trolling. I use mine pushing at 0.5 mph up to trolling crankbaits around 2 mph.

I think the new 999 is just a more modern version of the 898 HDSI. I wouldn't go below that myself.

Wish I could advise on the 999 or 998 but I can't. Just don't know enough about the new ones to do that. Personally, I wouldn't have a "59" series simply because it is unusable with the 360. But not everyone will be getting one of those.