I do maintenance for a cluster of apartment buildings in central Minnesota. We have 10 total standard 199k btu water heaters split betwen 4 buildings. They only seem to last about 5-7 years before rusting through. Every local plumber tells me that we have bad water and that is all anyone gets.

I am tired of replacing water heaters. I am thinking that a high efficiency mini-boiler running a stainless steel tank indirect water heater or 2 would last much longer than the glass lined steel tank water heaters my guy has been putting in and I would be able to replace parts instead of the whole thing. Any thoughts on what would last and not cost an arm and a leg?

Why don't you simply have the water tested and treated? Water isn't 'bad' by itself, it is the contaminants within it that cause the problems.

It is softened, are there other treatments?

Depending on what is in the water, yes. Also, I would turn off the softener in the first month of a new heater and let it build up a thin protective layer on the inside of the tank. That will inhibit corrosion that destroys the tank. Did you ever replace the anode rods in the other heaters? Soft water doesn't produce the protective layer of calcium, and will continue to eat anodes.

High efficiency boilers are sensitive to water quality issues also. I think stainless is the way to go. As far as price....." Pay me now, pay me later". If the price is double, and you can get 4x the longevity, it's a homerun.