I've only used the Tunze ATO. From what I can tell, the thing that usually goes first is the pump. I bought my Tunze used and it went for 5 or 6 years before I had to replace the pump. The Tunze has some electronics so these will eventually go bad but mine has been going for probably 10 years. Also has a power supply so that can always go out. So, I guess what I'm saying is it will depend on what you are looking at and how complicated it is and how much it relies on electronics. I'm an admitted Tunze fanboy so that is what I will always recommend.
I would think so. I bought my first SmartMicro ATO for my 90 gallon. I'm in the process of setting up a second tank and want the same system, I'd have no problems buying a used one given that the sensor didn't look beat up and I was able to plug it in to make sure the pump came on.
If it was a homemade setup, I'd skip everyday. They are selling that for a reason.
For me an ATO is not something I would cheap out on. Because I don't want a flood.just in case the pump don't shut off in time. I am all for saving money in this hobby but that is one thing that I will buy new IMO.