I would not be banking on either of the autotrol or sears unit working for a long time..
First look at the water test when it comes back.. iron and hardness and ph ,, that kind of thing. then how many people and number of baths and bedrooms for the house... as that will say what size unit is going to be needed.
If a system is cleaning say every 5 days there should not be much of if any issue with the brine discharge, but if say 24lbs every day or 2 times a day then that will most likely be a challenge for the long run.
There have been a few studies on brine in the septic.. mid west university's have done them.. look them up.