I use line counters and Dipsy divers for trolling (no need for down riggers) and it works well but that sounds like a really good deal. As others have said you can buy line with multi colors on it so that you can gauge your depth, the color changes every 10 feet or something like that. They'll work for you.
Dipsy divers put lots of load on your gear. You need to up size tackle to use them! Dipsy divers have their place, kokannee fishing isn't one of them.
I use been using Okuma Magda 20DX reels (which are pretty cheap) and number 1 Dipsy's on Erie for walleye's for a few years without any tackle issues at all, but that's just me...
ETA: I know nothing about kokannee fishing so I'm sure you're right about their application there, I'm just relaying my experience.
