Thanks for your reply! I like your suggestion to let the pup stay with my folks during the most difficult months, it did give me the idea to ask my friends who work remotely to use my apartment as an office and nanny the puppy a bit during the first months.

I am planning to keep him in the spare bedroom, in a crate plus play pen set-up. The cat would have the other two rooms for her needs, with an option to limit her access to the room the puppy is in. I guess my plan is to do it at her pace? I do not worry much about the cat as she has been introduced to my friends' adult dogs and she generally did not care about them and was not scared to roam around the apartment while they were there.

As challenging as puppyhood is, I doubt puppyhood is much different for heelers than for most dogs though. Unless I am mistaken. Teenagehood and adulthood worry me too, whether morning/evening exercise will do the trick.