Now I can't stop thinking about this idea. It bothers me that I can't easily be there myself, since I don't live there, but that doesn't have to be a show stopper?
It depends, to me, that would be a "show stopper", if you don't have physical access to the miners or/and have someone else who does, things won't be easy, you need someone who has basic IT knowledge to take care of the farm, while most operations can be handled online, physical attendance can't be avoided.
1. Is there a current, particularly recommended startup guide I can use?
Depending on the size of your farm, you might need to hire professionals to set-up the farm.
2. As soon as the miners are powered in Kazakhstan and connected to the router network/the internet via a LAN cable, I can do the configuration via e.g. Teamviewer from Germany, right?
Yes, you can do "most" of the operations online.
3. As far as I know a strong internet is not necessary, but a stable one. But what if the connection is briefly interrupted? Does it reconnect automatically? Can I reconnect from Germany? Or does someone in Kazakhstan reconnect the Miners?
It depends on the miner's firmware, some will auto-connect, others will need a reboot, both can be done online so in most cases there is no physical reboot needed.
4. What if the power connection briefly breaks? I'm afraid that someone in Kazakhstan will then have to push a button to get the miners to restart?
At its basics, yes. but there are some methods that involve the usage of some contactors/relays and other things that would minimize the need for someone to be there, but without a doubt, you can't completely eliminate the need for someone to be there when needs be.
5. It is very cold in Kazakhstan in the winter, but in the summer it can easily get up to 35°C. Is general cooling with fans enough, or do you need proper air conditioning?
You will need extra fans mounted in the farm walls, some for exhaust some for intake, again, depending on the size of your set-up you might need to hire a professional for the cooling system, no air cons are needed as long as you move enough air.
6. Which miners are recommended? According to Alibaba.com, you can buy a supposedly new Antminer S9 for $78. The ratio between power consumption and hashrate isn't great, but at this low price, it will pay for itself very quickly. Or is it too old and better to buy an Antminer S17 pro or S19 pro?
1-Do not buy from Alibaba,
2- There is no easy answer to this question, you need to do more research on your own, it all depends on the power cost there, if it's pretty low, you want to focus on cost per terahash, if not, then efficiency will be as important as the cost per terahash.
7. How long is the average lifetime of a miner? Or are there no reliable figures? Is 2 years too optimistic?
It depends on the gear, your luck, temps, and many other factors, it can be 4 years or 4 weeks, nobody has a definite answer, but it's safe to assume that if you don't ROI the miner in 6-8 months, it will be a bit hard to ROI, all the way to 1 year is somehow manageable, past 1 year and you are going to need a lot of luck.
8. How often and intensively is the necessary maintenance work during mining? Can the miners be left unattended for weeks or months as long as they are mining 24/7? Or should they be checked on daily?
If everything is perfectly set-up you might need to visit the farm for months or even years, but issues do happen, you also need to do some cleaning once a while, this could range from once a week to once a year depending on the location, filtration and all that.
