This question is a reach I know, but printing 1 million impressions a year, what might be the ballpark average yearly maintenance costs on a quality pre-owned Heidelberg speedmaster?

I would recommend contacting Heidelberg USA for this one, being as they have a complete line of technicians that specialize in their used machines. I know it has to vary greatly, as I once worked on a 2 color MOZP that a shop had just bought from another shop and it wouldn't register between units 1 & 2 especially up to speed at 10+. Finally after much frustration on my part and the owner finger pointing to me for some messed up jobs I threatened to quit if they didn't get Heidelberg in to find the problem which the owner denied even existed. The tech came out and within an hour found the problem. The machine had been crashed converting it from straight print to perfect or vise versa. I asked the owner about it and they claim to have not used it perfecting at all. To make a long story short it was an $11,000.00 boo boo, and the owner couldn't afford to fix it and said live with it, after a few more really long days of running the press slow to compensate I found another job working on a press that would hold register.

Very frustrating when you got to run a press that does not hold register!! Been there...done that!
Years ago we bought a brand new 4-color Heidelberg that would not register while perfecting. Seems Heidelberg was tired of coming down and throwing parts at it and offered us a good trade-in for a new 8-color. We took it, and it's been all good after that.
Good luck with your new job.