Apparently, sorting by a post meta will not work if the meta contains spaces...
It seems to be a convention to start all post metas with an underscore and separate each word by an underscore, too...

What is still missing is the detection whether an order is new or not. Old orders without ordernumbers should NOT be assigned a new ordernumber... No idea how I can find out this aspect

Initial commit, raw plugin structure, assigning random order numbers and retrieving them again works...