I have an ATI DP 6, it has six dosing heads. I have ran it for 3 years without a single issue with the pump (knocks on wood). 6 heads was more than I need, but I would recommend at least a 4 head unit.
I did manually dose my test for a 15 years before I got an auto doser. I was pretty good about testing every morning and never forgetting to dose. If you have a routine it can work. For me, I have never left my house without a cup of coffee in the morning, so I would make coffee, test and dose the tank and when I was done with that the coffee would be ready. An unexpected hospitalization of my wife and me staying at the hospital with her crashed my reef in 2 days; so that sucked.
With the auto doser I am terrible about testing daily, but I still keep on it at least once or twice a week, which based on my coral growth this far (knocks on wood again) seems to be sufficient. I do an old reefers trick of keeping a digitata in my system, and if the polyps are retracted I know there is an alk issue.
The only issue I have ever had auto dosing is when one of my lines got wet and the alk line clogged. I was out of town and my wife was testing daily, so we caught it on day 2 but it can happen. I am currently dosing Two part, zoo plankton, and amino acids. I do dose vodka for stretches of time when I have nutrient issues as well, but haven't in awhile.