Which DLCs to buy? Since this question appears quite often, I decided to provide a quick overview.
 
 

Why do I even need DLCs?

The base game itself is quite lackluster, unfortunately. It is nice, of course, but has low replayability. You may squeeze out a few more playthroughs by raising the difficulty level, but sooner, rather than later, you’ll get sick and tired of yet another dust storm.
 
 
You see, it’s Paradox’ policy to release a “teaser” of a game, then milk the players with countless DLCs. Surviving Mars is not quite as bad in this regard (at least compared with CK2 or Stellaris), but it still needs some of the expansions for a proper experience.
 
 
 

The DLCs aren’t cheap, though.

Well, you don’t need them all. Long story short, the Green Planet expansion should be the bare minimum which to buy, and it’s the only DLC actually worth its full price. The rest are nice, but not a necessity.
 
 
If you’re really strapped for cash, then I’d say wait for a sale, and maybe get the Space Race DLC as well if it’s 50% off or less. This will double a single playthrough’s length, and also provide more (much-needed) replayability. If you still have some money to spare, get the Colony Design Set and In-Dome Buildings Packs, too. The best decision, however, would be to wait for a package sale or a HumbleBundle that includes it all. Just make sure to deactivate Below&Beyond.
 
 
 

Long Story

1. Green Planet expansion. This is certainly the best expansion that greatly enchances and prolongs the game by providing a long-term goal. The terraforming of Mars is a complex, but rewarding process. Otherwise, everything becomes rather boring and repetitive after Sol 100 or so.
 
 
It’s the only one that’s worth the full price.
 
 
2. Space Race expansion. It adds rival colonies, unique buildings, vehicles, new factions, and a trade function. You will also have to compete for the milestones with the other colonies now.
 
 
Nice to have, but not a necessity. Get it on sale, or as part of a bundle.
 
 
3. Colony Design Set and In-Dome Buildings Packs. Those DLCs add more buildings and a few skins. The balance varies, the retirement home and school spire are too OP, while the hospital and large oxygen tank are very useful (and should’ve been in the base game).
 
 
With some buildings being useful, but others way too OP, it’s not a necessity as well. Again, sale or bundle.
 
 
4. Project Laika DLC, mostly useless. The ranches are an unimaginative, purely cosmetic “alternative” to farms, and the pets are just plain annoying.
 
 
Not worth it to buy individually, but if it’s part of a bundle, then why not.
 
 
5. Below and Beyond “expansion”. Just stay away from that piece of crap, its buggy as all hells combined and requires ridiculous micromanagement efforts.
 
 
Just don’t buy it. Even if it’s part of a bundle, deactivate it. Neither the “official” nor the community patches can guarantee that it works properly. Maybe in a year or so?
 
 
6. The various cosmetic DLCs are entirely optional, and add purely superficial stuff like radio stations or dome/building skins.
 
 
Just wait for a bundle or simply ignore them, unless you really need that little bit of eye (or ear) candy.
 
 
7. The newest “I like trains” DLC. Honestly, with all the limitations of flat terrain only, no elevations, no hub stations, too much micromanagement, it’s still too unpolished.
 
 
Either it gets upgraded to a level where it’s worth the price, or just get it on sale.
 
 

 
 
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