You could create a very nice collection of similar material without breaking the bank. Some of the pictured items are pretty rare, but some are pretty common. Historically important covers would be potentially pricey, but you can easily get some feldpost covers to capture that flavor if the idea of the covers is attractive to you. Otherwise, postmarks on piece, or on memory cards, are pretty easy to find.
The Laibach overprints got me drawn into this area. The Italian stamps are filled with overprint possibilities, which that link only began to hint at.
All that being said, there are probably a few caveats.
There are some tricky forgeries and fakes. The fact that there are plenty of Hitler heads floating around, coupled with the quick and dirty nature of some of the late war overprints, makes expertizing difficult. Some of the other issues were reprinted off of the original plates, which also complicates a positive identification.
The various legion issues are plentiful on
ebay, but I've found a few of them much cheaper here and there through other sources. It seems like this is one of the areas where dabblers in stamp collecting seem to hang out. That leaves one with sellers who assume things are rarer than they are, and buyers who will pay high prices because of the hype.
Buying or selling this material online will bring some interesting characters out of the woodwork. Expect the occasional message from both ends of the continuum. The far ends...
Good luck if you decide to head off in this direction. Share your findings.
Cjd