Everything below is my personal opinion and represents only my thoughts about the extra content of the game :)

None of the cosmetic DLCs are "must have" unless you really want to use a specific portrait \ emoticon \ phrase from that DLC. Personally, I like them all without any preferences but also would survive without them (a perfect example – here on GOG I have only the base game and have no intention to activate any DLCs). If you aren't sure if you want them or not, ignoring them probably would be a better idea. Definitely don't buy them until you know that you like the game enough to justify these extra expenses.

OST is the real "hit or miss" here. It might be great if you are a fan of such music. I haven't seen almost any criticism in its address, only praises, but personally, I wouldn't buy it. Electro swing goes way down in the list of my personal preferences in music and I don't listen to it even for free.

I really liked Artbook though. Obviously, zero value for the game or gameplay but it was an interesting read considering the fact that I wasn't with the team at the very beginning of the development of this game. But again, it just tells the story of how the game was "born" and what were the first steps in design, graphics, and the ideology behind it. If you aren't interested in such purely "academical" knowledge about the history of the game, you lose nothing for not having it, and I obviously can't guarantee that you will like it enough to justify the price. In the world where you can buy a proper game for $5 it might be a tough call and mostly depends on how much value you see in $5 to spend them on something that you probably will read only once.

The three character DLCs –

Uninvited Guests
Automated Competitors
Astral Gamblers

– were created for Gremlins, Inc. and are compatible only with that product.

Gremlins vs Automatons does not support DLC.