My initial concept would be to introduce a 'health bar' to machines much like the health bar on tools. However, instead of having a fixed number of uses after which they are guaranteed to break as tools do, I would have machines carry a per-operation-completed chance of breakdown, the odds of which start at 0% at full health and slowly get worse as the machine accumulates usage wear. The effect of a breakdown would be the same as a failed wrench use. I would add the health bar of the machine to the right-click gui, as well as the inventory slot decal as with breakable tools. I would also add a number-of-completed-operations counter to the right click gui.
... Did you understand the basic Idea of IC² ? And before IC², IC ? The aim was to make tools more efficient, and UNBREAKABLE at the cost of the energy. Do a furnace break ? No. So should machine break, when tools won't because of what I already said ? I don't think so.
If you want to create economy on your server, the most efficient would be to create machine adding few advantages to player, like "energy", added on your hunger bar, that you could reload with, for example Shower (we aren't these dirty Steves using pickaxe and dying because of disease, we've got IC² ^^), Heater or others machine that could create something similar to our "Post World War Economic Growth". I think that would be easier.
But it has already been suggested/denied I guess.