JT, the moose hunt with your dad should be the first priority. That is a given, however I believe you can do both.
If the rifle is in proper working order, and fits as well as you describe, it will be one of the best investments you ever make, both financially and personally.
The moose hunt with your dad will be enhanced by taking the rifle along, and perhaps giving him the chance to use the rifle on the moose.
As you have already read in this thread, ammunition for the 9X56 is easy to obtain with a proper die set, 8X57 brass, and there is no problem using regular .357-358 diameter bullets in these rifles.
I will be pleased to send you forty once-fired Winchester brass to reform, and I would not be surprised if there is an old 9X56 die set sitting in a drawer or on the back of a bench around here somewhere. If not, you can get a new set from CH4D for $100US plus postage.
The 9X56 easily gets a 250 gr bullet loafing along at more than 2300 fps with sensible pressures and long case life, and there isn't any outfit that will do much better on deer, moose, or bear out to 200 yards. Especially when they fit properly, these rifles swing and point like an extension of your arm. You may end up using it for most of your big game hunting.
Just take a few moments, very few, and think about what you can do to make it happen. There must be something you don't need in the coming year that you can pass up in order to get the Mannlicher Schoenauer, a once-in-a-lifetime opportunity, and still go moose hunting with your dad.
Both you and your dad will enjoy the rifle and the experience for a long time. The cost of the hunt, the rifle, and the dies, will be pretty reasonable when considered over those years.
Best,
Ted