Which Outfits should I buy?

Solution 1:

It depends on the area you are going to face.
I bought and invested in Protector armor, which works well against melee attacks and it's useful in different cases. I found long range protections quite useless.

Surely it's good to have fire-protecting and ice-protecting armors in late game. Elemental defences are situational, when facing specific machines they are a must-have in harder difficulties. As a side note, I prefer to maximize a singular element, instead of using more general mods. It's better to do something at best.

As @Mad Scientist is asking, the DLC area is strongly based on ice element, but fire-based machines are pretty common, too.

Solution 2:

There is an armor that you can't buy that is very strong on medium or lower difficulty levels (it is intentionally nerfed on higher levels). For me this armor did obsolete all other armors and I used it exclusively once I got it. You can get this armor once you progress further in the main quest, it is roughly 2 big quests from where you are now.

Spoiler for the exact quests you need to do:

You get this armor from the Ancient Armory side quest, and the last two power cells are in the Grave Hoard and the GAIA Prime facility.

Spoiler for what the armor provides:

The armor provides a shield that absorbs damage and refreshed automatically. It provides a lot of protection compared to the non-DLC armor with crappy mods, and also is a big boost to convenience as you need far less healing and herb gathering. It is significantly weaker on higher difficulties where it'll absorb only one hit usually.

If you use stealth, the stealth armor is still your best bet. When I played the game first, falling back to stealth was typically the way I resolved situations I failed at with brute force. For combat I prefer melee protection, for me when a machine got close enough to hit me that was generally the most dangerous situation. I found elemental damage protection too situational, so in the end I used the stealth armor with melee damage mods.

My impression is that unless you farm high-level mods and are in new game+ where you get an additional slot, the differences in armor are not that big until you get the armor I mentioned above.

The DLC added some more powerful weapons and armor, and a few very powerful non-random mods. It also added some enemies that are far more difficult than the ones in the main game, but you can do quite a few side quests in the DLC without hitting too hard of a difficulty barrier. In the DLC there are some very strong fire-based machines (and some ice-based as well), for these armor against fire is quite useful.