Many thanks for all this answers.

My first idea was ordering new devices. But I read a lot of stuff since and there were a lot of people that had trouble with their vendors. So I decide to buy used ones because that gives me the option to check the hardware before paying.

The cost for my power is really that low. In fact, I'm thinking about rent out the location for 20 % of the bitcoins that are earned here. That would decrease my income, but it would also decrease the risk. Maybe I do both.