I truly can't imagine maintaining an espresso machine in a commercial setting. I bought a La Marzocco when I moved outside of convenient driving range of any coffee shops, and it made great espresso, and I kept it in working order for 4 years, and then I just couldn't be bothered anymore and I drink aeropress or cold brew now. I care enough to drive two hours every month or so for green beans and to roast them myself, but keeping the La Marzocco working was way more trouble than it was worth.
They are pretty stable if you keep them on all the time. Turning them on and off, heating stuff up, cooling it down, putting it under pressure and depressurizing them causes more wear and tear than using them and leaving them on all the time. At least that is what I'm told, it's all peasant lore and I do what I'm told.
What typically fail on these things? Is it bearings, reservoir hosing and valves, washers, things of that nature?
Gaskets on the group heads are going to go first. I changed mine out after one year before they had a chance to leak pressure. They are cheap, more like maintenance than repair. I think I'm going to need to replace a valve or seal on one of my steam wands before long, seems like it isn't sealing as well as it should. Looking at the valve I replaced I think it might be original, so around twenty years old. Puro cafe is mildly corrosive to metals, so every thing down stream of the group heads is going to go eventually. My roaster has the copper connector to his drain corrode and leak a few weeks back. Another shop that my roaster sells to had some kind of water sensor fail recently. I think old espresso machines are like old cars, anything could go. If there is no water filter on a machine and the water has any mineral content than it's going to fuck the machine up badly. I think it was ecib that sent me some photos of a machine who's boiler had more limescale than open space in the boiler. If you don't keep your machine clean you'll make terrible espresso and it could interfere with the general operation. Lots of equipment chit chat on my roasters blog, most of it about coffee roasters but a bit about brewing equipment. http://www.couriercoffeeroasters.com/wordpress/