The second-hand route should be a viable option, with caveats.
In the past, NI has been very successful at coercing users into upgrading, resulting in lots of redundant versions of Kontakt on people's machines. I have versions 2, 3 and 4 on mine.
You'll always need the latest version to use any current NI libraries - that's their secret to forced upgrades - but third-party developers tend to lag behind so as not to unnecessarily narrow their market. Consequently, many third-party libraries are compatible with versions 3.5 and 4.2, and many go back to version 2.
The Indiginus stuff requires Kontakt 4.2, which is going to be harder to find because so many 4.2 users did not upgrade to 5. Kontakt 3.5, however, is easier to find, and there are a number of acoustic guitar libraries that will work under 3.5. For example,
Pettinhouse has a good one that runs under older versions of Kontakt. (
Video)
The downside to buying Kontakt second-hand is you'll be stuck with that version - no upgrade discounts - and whatever libraries you can find today that work with it. Over time, there will be fewer compatible libraries. Don't worry about lack of support from NI, it's nonexistent anyway.