Mostly just as a matter of curiosity, I'm wondering if this version somehow retains the option to buy extra praxis kits and whatnot, outside of what comes with the content/weapon packs that are part of the season pass. Not that I'm itching for the option, but they're not listed as DLC on the gog page the way they are on the PSN store, and I'm wondering if there's some kind of in-game menu option to go look at those, or if they're just gone.

There is no in-game store, so no you can't buy them.

What you CAN do is add them yourself if you care to. Check out cheatengine.org and fearlessrevolution.com.