Quote:
the 1913 Parcel Post with XF centering. Lightly mounted
You are describing premium stamps. Why wouldn't you expect to pay the price that premium material merits? If you are asking, "Do special opportunities occur?" the answer must surely be "Yes," but they are like lightning - infrequent and unpredictable. If that was a buying strategy there is no telling how long it might take to build the kind of set you are seeking. But if you are young (have the time) and are willing to wait, then go for it. But be prepared to wince a lot over opportunities missed.
How badly do you need the set? Is it just for bragging rights to say you have it, or will it figure into an exhibit? If it is the latter, then there is some pressure to complete the set sooner than later. If it is the former, then the drive to get such a set is more likely to pass before you have reached your goal.
The chances are highly skewed that you will take a long time to get most of the set cheaply, and will eventually need to bite the bullet for a few final pieces.