1. What are the parts that break most often and for what reason?
It depends on the miner version, the most common would be the chips themselves, there is no exact reason as to why that happens, it just happens, and it happens more often on some gears than the others, it's also common for PSUs to die.
2. Can the breakage be avoided for adding UPS or voltage stabilizer, etc?
Unless your voltage is out of the specified range labeled on the miner's PSU which is usually (220-240) it makes no sense to add those, or at least, the cost outweighs the benifit.
3. What is the easiest way to get spare parts and are they expensive?
Buy them online, from places like Alibaba or Zeusbtc, some are cheap some are expensive, but that should be the least of your concern, the question is, will you be able to fix it yourself? if it's just a matter of a dead PSU or a dead fan, anyone can replace those, if it's a bad chip then you will need a lot of skills and tools to fix it.
4. What would you say would be an average lifetime for a miner and can this be extended (does it make sense cost wise) with spare parts?
It depends on the model, Some S9s have been mining non-stop since 2017, some S17s died after 2 months, it all depends on the quality of the miner and pretty much your luck, of course, keeping them clean and cool will at least in theory give them a longer lifespan, but the things you can do to "extend" their lifespan are pretty limited to taking good care of them, there isn't something you can buy and install on them that will magically make them live longer.
