Is it possible to create a sensor device which contains the minimum or maximum of other sensors?
Yes, it is possible by using a VariablesDevice as suggested by @leader21. However, instead of using rules to assign the variable you can use an expression as part of the VariablesDevice - like what you had in mind
See working example below:
"expression": "min($temp1.temperature, $temp2.temperature)",
"name": "Minimum Outside Temperature",
EDIT: The resulting variable is $minOutSideTemperatureDevice.temperature in the given example. So, you can use this variable as part of your rules. The variable expression is evaluated each time when one of the arguments values (input temperatures) changes.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law