Yeah, it can be confusing because AT used to be a big all-encompassing software package that people bought, and then it kind of added/semi-switched to the "app store" type model.
I think you can actually still buy the "AT MAX" bundle in a physical cardboard box! Not sure either or both of the "AT 4 Deluxe" and "AT 4" bundles are download only. Then there are all the brand/artist bundles which I'm pretty sure are download only these days (though I could be wrong).
In practice, I think it's fair to say that "Custom Shop" is the basis of AT these days. Even if you bought and installed the MAX bundle, I think you would end up buying any subsequently released new models through the Custom Shop anyway.
So, minimally, one installs and registers "AmpliTube Custom Shop", which should include both the standalone app and plugin versions of AmpliTube 4 itself (specifically 4.6.1, at the time of writing!), as well as the separate "Custom Shop" app. The gear models bundled in the free version of AmpliTube 4 (= "AmpliTube Custom Shop") are a bit "long in the tooth" but still eminently usable. You can then buy individual items or bundles through the Custom Shop app as you will, and use them in the standalone/plugin versions of AmpliTube.
As DarkStar also said: keep an eye out for sales (like the current "Summer Strummer" specials), which are a good way to strategically expand your arsenal when you haven't got a lot of more up-to-date models. Also, make good use of the ability to demo gear before buying! It may take a while to get to grips with all the "flavors" available to figure out what you best like and what's of the most use to you.
Although I had been familiar with earlier versions of AT that friends had, I think I jumped on board with AT 3 when Custom Shop launched. I think a band guitar-tech on the old Amorphis website forum tipped me off to it! I think I picked up the Hendrix, Metal, and (when it came out) Slash bundles -- mostly through sales -- even though I'm not a raging Hendrix or Slash fan, or really a metal-player by contemporary standards. There was just a lot of useful gear in those for me! Otherwise, I've kind of cherry-picked.
Does the Custom Shop had a "wish list" feature? It should. I often demo things and end up thinking, "well, I can't really spring for that now, but maybe if I get a little extra next month ...". Then -- if I end up feeling a bit more flush -- I could quickly skim the "wish list" and say, "Oh, yeah, right, that amp was just what I needed for that other song I was working on ...".
